I read about this issue some time ago, and I find it idiotic that because the traffic is being used for a purpose that the ISP doesn't find "fitting", they find the need to throttle it back to snail speed. Big deal, its data packets - if you don't want users using your service for such a task, then clearly state it somewhere. Not difficult.
It's not really that they don't find the use fitting, it's that p2p generates a lot
of traffic, and does so constantly. Many ISPs oversell their capacity, but it isn't that
much of a problem with normal
users who have relatively short bursts of traffic, and does the occasional large download.
But once many users start doing constant speed-saturated stuff like p2p, these greedy ISPs get into trouble because of their overselling, and thus they like to throttle p2p.