"If you want to use Intro.js for a commercial application, theme or plugin the commercial license is the appropriate license."
...It's not saying you have to purchase a licence if you want to use the software commercially, but that the author recommends purchasing the commercial licence, which they are perfectly within their rights to do.
There is absolutely nothing stopping you from using the software under the terms of the MIT License, but in that case you will have to abide by the terms of the licence and include the copyright and permission notices when re-distributing (the commercial licence may or may not have these requirements -- I haven't seen it).
According to the Intro.js website, the top two tiers of the commercial plan includes support and code review [2], which aren't included with the MIT License.
Depending on the situation, one licence may be favourable than the other. It's not too dissimilar to MySQL being available under a free licence as well as a proprietary licence -- it provides a choice. At least, that's how things are as far as I can tell.
"If you want to use Intro.js for a commercial application, theme or plugin the commercial license is the appropriate license."
...It's not saying you have to purchase a licence if you want to use the software commercially, but that the author recommends purchasing the commercial licence, which they are perfectly within their rights to do.
There is absolutely nothing stopping you from using the software under the terms of the MIT License, but in that case you will have to abide by the terms of the licence and include the copyright and permission notices when re-distributing (the commercial licence may or may not have these requirements -- I haven't seen it).
According to the Intro.js website, the top two tiers of the commercial plan includes support and code review [2], which aren't included with the MIT License.
Depending on the situation, one licence may be favourable than the other. It's not too dissimilar to MySQL being available under a free licence as well as a proprietary licence -- it provides a choice. At least, that's how things are as far as I can tell.
[1] https://github.com/usablica/intro.js/blob/master/license.md
[2] http://introjs.com/#commercial