Everything that shows up in a multiplayer match needs to be on both ends of the connection, even if one player hasn't paid for it. Would you rather everyone go download a bunch of compatibility packs (like Mortal Kombat) just to get online at all?
The larger problem is that some of the DLC gems are better than the default gems, which is something they said wouldn't be the case.
I guess gems shouldn't matter (don't know what they are), but I can understand the complaint against DLC characters. If it was made before the games release, then why sell it? Sounds like a greedy way to make a quick buck IMO.