AT&T does. My internet would become unusably unstable every time I torrented, so I called them about it. They said they blocked torrents to "comply with the law" or somesuch BS, nevermind that I was attempting to download a Linux distro.

I didn't say they didn't, just that they weren't allowed to. See e.g. http://money.cnn.com/2015/02/05/technology/fcc-net-neutralit...

You may have also been hitting this: http://en.wikipedia.org/wiki/Bufferbloat

I wouldn't put a lot of faith into what front-line tech support says. They'll often outright lie to you to get you off the phone.

Can you elaborate on this? My current ISP definitely shapes torrent traffic to the point where it is impractical to download anything larger than a few KB.