This is simple. They are ether common carriers or they are not.
If they are common carriers then they can not inspect the content they carry and as such are not liable for that content.
If they are NOT common carriers then they can inspect the content and charge what they like. However, they are liable for the content they carry. Thus if they choose to not be common carriers and someone is transmitting Child porn, threats, selling drugs, pirated music and movies,etc. Then they are liable for the transport of that data and we should prosecute them for it.
Do it just a few times and all the ISP's will be on board with becoming common carriers.