Because it's business my friend,sony have done it themselves in other ways,it's a competetive world,it's just the way things are. In topics like this i agree we should all get DLC at the same time. But untill bethesda or other companies stop,we won't. Be it sony or microsoft each one will want an edge. Like i said though,why don't sony do the same regarding DLC?
Why should Bethesda even consider doing this unless Microsoft paid them more than their estimated income from sale of the dlc on PS3? Far more as this piss of customers
As I guess they sell around the same numbers of dlc on Xbox and PS3 and Microsoft get less than 50% on the profit they this will cost them money and I doubt it will increase the sale to compensate for it.
A first release on one platform and a later release on another is more probably, however this will also cost MS money.