why don't free systems support mp3?

The reason many free systems don't support MP3s is because of software patents (which are simply a bad thing; that software patents are no good is universally recognized by software developers). Because the format is patented, distro makers that include the free software that can play MP3 files could be sued, so often they choose one of two options: either they choose not to support MP3 at all, or they include nonfree software which is safe for them to include (because it's been authorized by the patent holders).

Trisquel, however, is developed from Spain, where there are no software patents, so the free software for playing MP3s is included.

That's my understanding of it, at least. If I'm wrong, someone please correct me.

MP3 is protected by some patents. Some systems (Fedora for example) in their software guidelines want to avoid programs that have parts covered by patents and so do not include mp3 players.
Trisquel, which is based in Spain, has no such issues and supports mp3 by default.

Thanks guys. Armworm, I don't have a specific issue; I was just wondering because I noticed that some systems don't include an mp3 player by default.