Re: Why the hell...
'Lots of other ISPs are unlimited though.'
Technically this isn't true. There is an implied limit imposed due to network speed. So if you're on a 20mbs line, then that's your limit: 20mbs. Can't go paying for a 20mbs line and expecting to get 120mbs, now can you?
Again, technically not true: You have a limited timeframe* and a limited connection speed, which means there is a cap on what you can upload/download**. These are even artificially enforced as the line might be able to handle 30Mbs or 100Mbs but you're only paying for 20Mbs so you get 20Mbs.
It all comes down to how you want to interpret 'unlimited' and 'cap', but it isn't fair to say that one service is unlimited when it admits it throttles the connection speed at times and under certain conditions while others could well be throttling their service all the time, or doing it without warning you.
If any of these ISPs were serious about fair use, they'd work out how much you'd used and refund you an amount if you had used less than x amount of the service you had paid for (much as BT were doing on some phone tarrifs).
* It's easier to work out the cap as an amount over a given time, such as a day, a week, a month or a year. The cap is generally more than you'd ever reach, but it is still there. See **
** a rough calculation puts the cap at 1,728,000 Mb a day for a 20Mbs service
*** And with that pedantry out the way, I'll go get my coat. It's at home...