When 'limits' aren't really limits
In one respect, using the term 'unlimited' is patently false, as data rates are finite, and the length of billing periods is also finite, and the combination of the two imposes a limit on total data. They might be trying to say that they don't impose a limit on data quantity, per se, but by throttling after a certain volume, they are doing exactly that.
The way to get around these semantic tricks is to reword the contract to say 'unlimited time and volume at xxx rate.' Actually, some smart lawyers (I'm certainly not one, not a lawyer) could probably write a more binding, no-loophole contract that would deliver what the customer expects: a fixed and certain data rate for the entire billing period, with no data volume limitations. (Yeah, try running that past an Internet provider's billing and legal departments. Or go tilt at windmills... it's more productive.)