you don't get anything for free. I have a verizon 4gb plan, if i hit 4.01gb they add 1gb and charge me an additional $15.
If companies charge you X amount for every GB you go over, dont you have to go over an additional GB to get the charge?
Lets say you pay for 2GB...
If you're at 2.5GB, yes you are over, but not over by a GB