Actually, MS isn't forced to release the PC version. The only reason it is being released is that the guys Gearbox Software asked MS when Halo for PC was going to be released. They were surprised to find out that no one had been working on it and that it didn't even exist. So they asked if they could do a port and MS said sure whatever.
As for Halo 2, I'm sure MS will let Gearbox do the port again, and I'm sure Gearbox will be more than happy too. Of course, if Bungie isn't busy they could do it themselves.