It's not about writing code, it's about using algorithms. If you're using MP3 algorithms to make money selling phones, you have to pay a fee. If you're using MP3 algorithms to make money selling your WinAMP player, you have to pay a fee. Whether you found implementation on github or read patents and implemented it yourself, does not matter. I don't think that you have to obtain a permission just to write implementation out of curiosity, but I may be wrong. And I have no idea about status of open source projects which include patented algorithms, but distributed free of charge. Probably they can't do that, because Linux distributions did not distribute mp3 codecs by default.

Also all of this is applicable to countries where those patents are working, of course.