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.