I use AT&T broadband and there is nothing in the subscriber agreement that 
says I agree not to set up a sever of any kind.  What it says is:

AT&T broadband will provide customer with dynamic IP connection(s) as a 
coponent of the service. ... Customer agrees not to use dynamic DNS to 
associate a hostname with such dynamic IP connection(s) for any commerical 
purposes.  

Other services may differ of course.
"commercial" always seems open to interpretation.

Interestingly,  It also does not seem to prohibit me from setting up a sever 
for commercial purposes as long as I do not provide a dynamic DNS Link to it.

-- 
Paul Overby
xpoverby at ATTBroadband.com