Yeah, I have to say, I really don't see the problem. 250GB is loads. If they cut you off completely for going over that cap, then that is a bit harsh (throttling you down to like 25% of your normal speed, or charging for bandwidth used over the cap is much more reasonable), but I don't think you realise just how much strain heavy downloading can put on a network, especially an ADSL network (and especially with something like Bittorrent which uses hundreds of connections at once - it's not unheard of for a single bittorrent user with a badly configured client to completely knock out a router by, effectively, bombarding it with a DOS attack when they try to download, though generally the home router or client's NIC would die long before the ISP's does).
Even with a good network, there usually needs to be a cap at some point, because it's just not really feasible for any ISP to build a network capable of supporting lots of users downloading at max speeds at once. They need some way to limit the impact on the network and maintain reasonable speeds for everyone during peak conditions, and a cap is one obvious way to try and curtail excessive downloading, in order to achieve that. 250GB seems pretty reasonable to me, since it's something 95% of customers will never hit.
I used to have Virgin's 20Mb fibre broadband (I'm in the UK), and they had quite a good system for handling it. Rather than a hard cap (I think there was one, but it was ridiculously high, you'd have to try really hard to reach it), they outlined when their peak hours were, and said that if you downloaded more than a certain amount (10 or 15GB, I think it was) during peak hours on any one day, you would automatically be throttled to 25% of your normal speed for a few hours. It worked well, because it didn't stop people from downloading lots of massive files if they wanted to, but it forced heavy downloaders to think a bit and try to do big downloads at times when fewer people were using the network.
