That's passively preventing. (YouTube lets you share to Facebook and Twitter, so the lack of this ability from G+ is a lack of a feature, not one of Google's design goals. Code takes a long time to write and everyone wants a different feature. Eventually we will have them all, but right now, not enough code has been written :)

Prioritizing G+ over an open API (and a "Chinese wall" between the apps and the platform) is still preferential treatment, and is the sort of thing that got Microsoft consent-decreed.

I am not saying Facebook is better, but you can't say Google is better, either.