Because they can nail you on a TOS violation.

A web server is no different from a SSH server. Many of the edge devices the ISPs provide have them built in.

That changes nothing about it being a TOS violation.

I don't particularly agree with it, but thats what it is.

The point this parent comment is trying to make is that they're both technically ToS violations (both are running servers), but with the SSH server, there's a whole lot of traffic that the ISP could do something about (the botnets), and the owner would love it (not having my network constantly attacked would be good).

If an ISP is concerned about traffic volume, take out the botnets hammering away at the doors — the occasional guy's private website getting slashdotted is small, temporary potatoes.