Oh. Because of the special features.
(AVCHD doesn't have special features.)

That's a lot of extra stuff :)

Still, if someone only wanted to use the m2ts, they could tell their client to download only the m2ts and nothing else. Which, again, doesn't require it to be separate.

All I know is that I'm going to keep a backup of the whole thing, whether or not I need just the m2ts to encode, and I would prefer to keep said backup in immediately usable form. If it just so happens that the smallest (complete and burnable) version of 2.1 available is 23GB, then so be it.