MP3 decoding is just math. If you have enough CPU to decode a compressed audio stream, it doesn't cost anything to also have it decode MP3.
Sure, you could store those clips in a more appropriate codec, but what do you gain? It's dead simple to encode an MP3, every audio editor from the last couple decades can do it. How common is SBC support?
If MP3 decoding is free, and MP3 encoding is free, why bother spending any effort at all on anything else? Sometimes good enough is good enough and cheap and easy wins over correct every time.
So why does that decoding chip have mp3 decoding, and why use mp3 when you could just use SBC.