Jetpacks aren't exclusive to Spartans, they're used by Marines in Reach and mentioned several times in the books as a means to move around in zero-G, there would definitely have been some aboard the The Dawn at the end of Halo 3.
As for the other changes to the armour.. well.. how're they gonna sell a new range of action figures if they look exactly the same as the old ones?
The armour changed between Halo 2 and 3 and was never explained, I don't think it's worth getting hung up on. (Obviously some people will say it's game breaking and that they'll never buy another Microsoft product until they patch in the correct armour, but those people are MENTAL.)
Personally I'd like the jetpack to be a permanent feature of the suit, do away with Armour Abilities and just have jetpack and sprint as ever presents.