Data caps allow people who consume less bandwidth to pay less money than those who consume more. Its a fair system - it would not be if Grandma Poppet had to pay for unlimited b/w for the 7 emails she gets each month whilst Salty pauses his 24x7 Steam downloads only so he can stream his nightly movie marathon.
(OK, in hindsight, my parking gge analogy doesn't really work all that well).
you are right that this is probaly one of the main reasons for having different sized data caps. it allows price discrimination.
Most people think of price discrimination as a bad thing, but it can be very good.
for one thing, it enables people who don't want to spend loads of money on broadband to still get access.
If there was only 1 plan, unlimited data with a landline (for easy comparison), the price would have to be somewherein the region of $85-90.
But with todays plans, you can getplans for $75, or even less if you think flip have a sustainable price structure at $50 (they don't, based on their submission for the comcom). those plans would simply not be possible without the people paying more subsidising the other people paying less.