
The telephone company has the right to manage traffic on their network too, but if they tried to do it by getting you to use your phone less, by calling you up with the spoofed caller ID of a friend and faking a message in their voice telling you never to call them again, you wouldn't think it anything less than completely clear cut.
The examples you gave /don't/ muddy the waters all that much. Transparent http proxies don't alter the content. If they /do/ alter the content (such as Rogers have been doing lately), it is seriously controversial, as recent events have shown.
So the degree of acceptance toward ISPs' impersonation practices that currently occur is _predicated_ on them not *interfering* with your use of the service nor *falsifying* the data through abuse of their position, which is essentially that of a man-in-the-middle. Conveying our packets is a very serious social and moral responsibility, like conveying our postal mail or phone calls, and tampering with either should be equally taboo.
Re. the technical difficulties argument: I'm not buying that one either, or only marginally convinced that it couldn't be addressed pretty straightforwardly using existing technologies and only a modicum of development and deployment effort. If you can spot a p2p syn+ack packet flying across the network and send a rst, you can just as easily slap a tag on the traffic and shunt the punter sideways onto a lower traffic priority vlan, or I would like to know why not?