Second, variable pricing makes sense if those who use very little data get a discount off the amount this currently use.
* in the US, others feel free to convert to your local units.
Pricing data in MB (or KB!) is like billing for water by the cup (or Teaspoon!).
If you require someone to perform multiplication beyond 10^1 scale to estimate costs, it becomes easier for telcos to overcharge users. You and I probably have no issues with multiplication in our head on the order of 10^2 up to 10^6, but for most users, that scale is mentally taxing.
Compare:
0.02 cents per sms message $20 per 1000 sms messages
Plus, prices are "stickier" at 0.02/message than 20.00/1000 messages, since it's easier for two telcos to start discounting their price relative to one another with the latter, like $15/1000 vs $20/1000. The equivalent to such a discount under the per message pricing schema is to charge fractions of a cent. i.e. 0.015/message vs. 0.020/message. The only place in society where we really see fractional cent pricing in daily life is with gallons of gas, and even there the burden for the end-user is high enough that the gas stations all get away with leaving that fractional amount at nine tenths of a penny almost all the time.