E.g. for Fedora, it is in rpmfusion repository and not in the distribution proper. Other distributions have similar arrangement for license-os-ok-but-patents-are-problem situation. These servers are outside US (or other countries that recognize software patents), and for the US users, the issue of obtaining the license is up to them.
The situation is so bad, that Fedora stopped shipping support for hardware acceleration of patented codecs (i.e. not complete codecs, but support to use the implementation in hardware, for example in your GPU), because they could be sued for contributory infringement.
Also note, that binaries for VLC or ffmpeg for Windows or Mac are similarly distributed from non-US servers, so basically the same situation as rpmfusion.
I may be misspeaking about FAAD2, but I've never run into an open-source player (like VLC) or library (like ffmpeg) which hasn't supported at least HE-AAC decode for a decade or more. If that's wrong, I'd love to be corrected in the most detailed way possible.