While legality may or may not play a role, for the combo headsets it's just not technically feasible to put the monaural sound coming out of a phone through both ear buds.
Here we have a picture of a mini plug (actually it's full size, but it'll work for out purposes).
<img src=http://www.bonnersmusic.co.uk/acatalog/AC-PMSJFS.jpg height ="10%" width="10%">
Note the two black bands. The metal parts on the end of the plug and in between the black bands are where the connection is made. Monaural plugs, such as the ones used specifically for phone headsets, only have one band. That's because there's only one connection in a monaural headset, whereas there's two in a stereo headset, one to carry the left channel and one to carry the right. Thus when you plug a stereo plug into a monaural jack (such as the one on your cell phone) only one connection is made and only one ear bud gets sound. It is possible to add an inline switch that only goes through one channel in one position and goes through both in the other, but I've never seen a headset like that out of the factory and it'd be more trouble than it's worth to DIY one.