Note that once you get that source code, when you start using it, then you are responsible for the patent royalties.

I think this is all left intentionally vague by patent owners, since they want you to just pay to be on the safe side, there's plenty of straightforward things you technically can't do with patented codecs, but which you'll never get sued for e.g. the person you pay to video your wedding has no licence to sell you the H.264 file the camera produced, and no way of getting one.

So I think it's at least partly based on what they haven't bothered to sue anyone over, though perhaps free speech comes into it in the USA.

having source code has nothing to do with patents. its completely separate. likewie for the license. both could be public domain (no license) or licensed. in this case its licensed under the terms of the gpl v2 and a bunch if the code is probably patented, (again, compiled or not, that doesn't change anything)

You can study patents for free; that is part of the purported purpose of the system. So if you copy the code purely to learn about it, that is arguably not an infringement. Using the code is in violation without a license. This applies whether source, interpreted, compiled, bytecode, or decompiled. It will be harder to convince a judge that you were just studying it if you have only a form that is conducive to running and not conducive to study, though, if you are found out and sued.

Of course, you'd better not plan on using anything you learned from it until you negotiate licenses for your users (or the patent expires).