Are these icons free?

Although it does not use a standard license, it should be considered free (selling and modifying is allowed).

I also found some icons licensed freely. The 24x24px PNG version is licensed CC BY, but the vector and larger versions are proprietary.

http://glyphicons.com/

Hm... that license is a little too non-specific to me. They say you can "use" it in your "projects", but it's not entirely clear what they mean by that, and whether or not you can just distribute modified versions.

This is not a software, therefore it cannot be proprietary (they just selling/licensing them, which is perfectly ok). Proprietary means when the author release a program, without "providing the source code under liberal terms" (i.e. those respecting the user's essential rights - the four freedom rules). If the application became popular, there will be many users grow into, and depending on it (this is the main problem why the transition from proprietary software is hard most of the time).

Now, the problem is, that the author became some sort of "God" - can see the code/modify the application however he/she wish, even put backdoors into it, since the compilation process mask every trace of his/her mischief. The only neccessary task here is to ensure, that the application stay popular, so implementing new features (even if they doesn't benefit the users/make the application better), using new technologies, and making the overall look fresh and attractive is needed.

That's why you can see complex proprietary applications, provided free-of-charge (aka freeware), so that the companies make you/other people accusomed into their particular product. Companies needs to have control over the market, and people who use their "products", because information means power (and money).

Hope this help you better understand the issue.