Actually, anticompetitive, and restraint-of-trade come in as better arguments. They go along with blocking port 587/110, keeping users from getting at legitimate, well-run remote mail servers. The end user paid for packet service, and the Internet generally permits any protocol to be run. ISPs legitimately block traffic at various protocol levels to deal with security and abuse matters. That's unlikely with VOIP.
Blocking for dealing with security issues is one matter. Blocking to purposely harm competition is another, and will indeed open a can of worms if it persists.
The anticompettive argument is pretty dangerous. What if ISP A has VoIP serice offering and is provisioning QoS for their VoIP service. At the same time they are not providing any QoS for competing services or even degrade service quality for competitors via bandwidth management.
From the ISPs perspective it makes perfect sense to beef up bandwidth to offer VoIP to be able to carry traffic from paying VoIP subscribers. But it doesn't make sense to beef up bandwidth to support the competition.
What it'll come down to is a definition of what services you average ISP provides. What is internet access, a raw pipe to that indiscriminately moves any packet from point A to point B? Obviously not, since there are already restrictions on running servers, blocking of smtp, etc. So it is perfectly reasonable, IMHO, for an ISP to regulate bandwidth availability to please the majority of paying customers. Adi