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).
In that case somebody is using low quality equipment.
1) While BitTorrent may overload the ADSL link between my and the ISP, it will not overload the link between DSLAM and the rest of the ISP network. If it does, then the link is too slow for that DSLAM.
2) "hundreds of connections" is only a concern for the client, server and a NAT router or a firewall. Transit routers just forward packets, they do not need to concern themselves with TCP connections. So, if a ISP grade router got knocked out by a single ADSL user, then I think the ISP should not buy any more routers from that manufacturer (or stop buying consumer grade routers and using them as ISP grade ones).
3) When I had DSL, I had a router made from a slow PC (Pentium 120MHz, 64MB RAM) and "smoothwall" router Linux. After configuring, it could pass up to 20mbps (connection speed was 4mbps, so I could have used a slower PC if I had one) and maintain up to 4000 connections. It never slowed down because of BitTorrent and lots of connections.
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.
The need for a cap is a sign of too much overbooking. For example, my ISP overbooks 30:1 and it does not need any cap. The speed does not get that low anyway. For my 300mbps uncapped connection, the slowest I get is 40mbps during peak conditions. No cap at all.
Also, to allow regular users to go to google faster, the ISP can prioritize HTTP (and real-time protocols) over BitTorrent (and other p2p protocols). Then google will load faster and you won't mind that your BT download takes a few more seconds to complete. My ISP did that some time ago, but it no longer does that, I guess the network was upgraded to be good enough.
As for the 250GB cap, when I had DSL (4mbps down, ~800kbps up), I managed to upload up to 250GB/month and thought that my connection sucked because the upload speed was too slow.
I now have a 300mbps connection and upload more than that cap in a
day.
1080p anime series is about 23GB, so download a few of those, some HD movies and watch some HD Youtube and you'll hit your 250GB cap in a couple of days.
Usage stats for this month at the time of writing (my router can display them): 362GB down, 15148GB up.