No, a normal ISP does not prohibit running servers at all
because strictly speaking you're not an ISP if you do that
rather some kind of "online" service that partially overlaps
with the internet. You certainly won't be subject to such
terms when using a proper ISP.

They can hardly get more explicit than that. Don't think
they "allow" it because they use no technical measures to
prevent it. As with law and order, most things which are
illegal are not and cannot be efficiently be prevented.

However, regarding the original problem I would guess that
the user's router is too crappy for P2P. Some of them cannot
handle more than dozen connections at any time. Or the
user should just decrease the bandwith limits.

Furthermore, just because some people can use P2P applications
at comcast that doesn't imply everybody can. It's well-known
that certain ISPs ban P2P only in certain areas, at certain
times etc. That's either because they cannot technically
handle complete blockings e.g., it might require special
routers - there's is hardware available for messing with
P2P in several ways. Another reason might be that they do
this on purpose to prevent that anyone can easily prove it
and instead blame it on the users' software or hardware.