att at the time they were selling they offered BB data plan as optional and to use mms you only needed PPU (pay per use) media net data plan. That was in early 2008 when many new smartphones offered full benefits of 3G except of Blackberries until summer when Bold came out.
I understand not everybody was happy with BB limits so they offered more flexibility.
Now's that market changed and Blackberries are very advanced, obviously the strategy changed. BB data plan are now mandatory and no PPU, but now you can take full advantage of 3G service.