I recall when they added a small amount to the price of blank audio cassettes in order to cover the losses due to copying albums onto cassettes. Never mind that the quality of the copies made that way was inferior, the money was paid by everyone. I think they could probably get away with adding a $1 to $2 fee per month on everyone, but more than that would wipe out any download service that charged.
The problem with an ISP tax versus a cassette tax is that your ISP connection could be used for things completely unrelated to music. Sure, a cassette could be used to record your child counting to 10 for the first time, but at least it was audio-based.
With your ISP connection you can read e-mail, view web pages, take place in discussions, post photos, play games, etc. You can spend months online with your PC muted without any major loss in functionality. (YouTube's value would be much reduced but most online services don't count on audio.)
In fact, a mandatory ISP tax would force deaf people to pay for music that they couldn't listen if they wanted to. How is a deaf person supposed to "earn" their Music Piracy Fee?