It honestly sounds pretty convoluted and unenforceable, and with little payoff. How are you going to detect and sue someone for what they do on their own computer?
They can hardly successfully sue folks for torrenting. Text and image advertisements aren't affected by this, either.
As far as blocking video ads goes, you don't even need to access the video. EME protected video components still show their length in the browser. Blockers which never actually see the content are entirely possible by just looking for videos less than a minute or so.
>How are you going to detect and sue someone for what they do on their own computer?
MAI v. Peak is the case that proclaimed the "RAM copy doctrine", which says that loading a computer from disk onto RAM constitutes a new copy under the Copyright Act, and therefore may infringe.
The specific case was about whether Peak, a computer maintenance and repair company, had the right to load diagnostic software that the computer's owners had purchased. The Court ruled that they did not, and that by copying the program data from disk to RAM (that is, running the program), the computer technicians had violated copyright.
Congress addressed this case specifically with reference to computer repair by carving out a special exemption for repair technicians to run the software that the computer's owner had purchased. Said exemptions don't apply to anyone else, though the owner can "make or authorize the making of" copies if they are necessary to actually use the product (I don't know why the owner's authorization to make a copy wasn't good enough for Peak).
Practically speaking, of course it's unenforceable on the large scale because as you say, it'd be hard to detect who is doing it, and the more fuss they make about this type of stuff, the more damage they do to themselves.
As far as blocking video ads goes, you don't even need to access the video. EME protected video components still show their length in the browser. Blockers which never actually see the content are entirely possible by just looking for videos less than a minute or so.