I think the theoretical max speed for EuroDOCSIS 3.0 is 400/108Mbit or something.
At least back in the days, a common complaint about cable was that "the entire block had to share the same bandwidth" and that it usually created a bottleneck. Not sure why that wouldn't apply to every type of internet connection in the world in one form or another, but it's just what I heard back then.
If that still is the case (cable having bottlenecks, I mean) I guess they're afraid that torrent users would saturate the upstream. We all know how much traffic a popular torrent can generate if there is enough upstream bandwidth to meet demand. Meanwhile, a normal user will run out of disk space sooner or later regardless of how fast his download speed is.
If you happen to live in a building or a housing area where my ISP offers fiber, the pricing is exactly the same as I'm paying for cable... but with faster upload. You get 10/5, 24/5, 50/10 and 100/10 instead of 10/2, 24/2, 50/5 and 100/5.