Being able to see the source is not the same as open source. Being able to see the source doesn't mean you can fork it, resell it, or even use it without permission.
Plenty of proprietary products are made in languages where the code is not compiled, such as proprietary PHP platforms.
Presumably, since the plugins are free here, if Google isn't open sourcing it, they don't want other companies to lift their work and use them in competing projects. Or, just as likely, the Googler that wrote it has some bureaucratic process they didn't go through yet to open source it, and they intend to change the license later.
I'm not too familiar with open source licenses, but does a license exist that allows someone to (enforceably) open source their code with the stipulation that it is for academic/security purposes, but forbids forking/reselling/other usage?
They aren't open source licenses then, by definition. But there are definitely things comparable. Microsoft's "Reference Source License" is a good example: You're allowed to use their source as a reference for debugging and security, but not able to fork or redistribute any of the code: https://en.wikipedia.org/wiki/Shared_Source_Initiative
Sometimes these types of licenses are called "source-available" rather than "open source", which distinguishes the significantly different type of permissions associated: https://en.wikipedia.org/wiki/Source-available_software
Sometimes "open source" means "you can see the source code".
Sometimes "open source" means "meets the requirements for software licenses under the Open Software Initiative's definitions."
Similarly, sometimes "free software" means "provided at no monetary expense" and sometimes it means "meets the requirements for software licenses under the Free Software Foundation's definitions."
People really, really like to pretend that "free software" and "open source" are extremely simple concepts and synonymous terms. They're not. People like to pretend they're controlled terms and that the plain English meaning isn't valid. That's absurd. Neither one is particularly easy to understand because both are much more deeply nuanced than you see at first glance, and one look at free and open software licensing comparisons [1] will tell you that there is obviously significant complexity going on.
This was settled many years ago by coming up with an official definition of "open source" [1] that was then popularized. It's a well-known industry standard. While nobody can prevent you from using the term incorrectly, you're going to get a lot of pushback if you do due to the confusion it causes.
There are a lot of other possible licenses and just because they don't meet the open source definition doesn't mean they are bad. I disagree with the people who say the opposite of "open source" is proprietary.
That's not really what I'm trying to argue. What I'm trying to argue that you can have OSI open software that is not FSF free software and you can have FSF free software that is not OSI open software, and either of those types of software are labelled as proprietary by the parties that exclude them. My point is that just because something doesn't meet the OSI open source defintion doesn't mean it's proprietary.
However, if I do consider your argument, it's not very good. The argument would be that the Open Source Initiative is not the authority for the plain English meaning of terms. If the argument I'm making is that OSI's definition is not authoritative, you can't cite the OSI's definition as an authoritative counter argument. You're begging the question. Yes, people are going to assume "open source" means "OSI open software," but that doesn't meant that everybody is always using it to mean that.
Both FSF and OSI fundamentally recognize each other as "not proprietary", even if they have different goals and different preferences on how FOSS ("free/open source software") is licensed.
And the OSI is not the authority for the meaning of English terms, correct. But the developers who use those terms are (much as language is determined by how people use it), and by and large, most everyone understands and agrees that open source software is software that meets the qualifications set by the OSI.
I'm usually a descriptivist for most English usage, but in this case we're fortunate to have an official, useful definition and should promote it over others, rather than letting it slide into becoming a meaningless buzzword like "agile".
I'm not sure how common non-standard usage is yet but I don't think it's too late?
I think the accepted terms are "open source" (free software that may or may not be "copy left", eg: gpl, bsd, mit) and "source available" (proprietary, you can see the source, but not redistribute etc. This is a spectrum from old qmail ("no forks, verbatim copies ok) to tarsnap (code available so user can audit) and code available under strict NDAs (I believe Microsoft provided source to dod in order for dod to consider windows for secure systems, for example).
Not a licensing expert myself, but I'm sure that exists out there. Creative Commons has some pretty close variants, for example CC-BY-NC-ND (attribution, non-commercial, no derivatives).
EDIT: I do understand that CC licenses aren't really designed for code, and that these restrictions probably don't make sense under the label "open source". Just wanted to link a possible starting point.
Open source also doesn't necessarily mean any of those things. They can make the source code public on GitHub under a proprietary license, which doesn't let anyone reuse or resell it.
Plenty of proprietary products are made in languages where the code is not compiled, such as proprietary PHP platforms.
Presumably, since the plugins are free here, if Google isn't open sourcing it, they don't want other companies to lift their work and use them in competing projects. Or, just as likely, the Googler that wrote it has some bureaucratic process they didn't go through yet to open source it, and they intend to change the license later.