If it's legal it's legal, thats the end of. What are you suggesting? Lock up your example paedophile despite the fact he's broken no laws? If he's broken no laws then he's broken no laws, despite people's opinions.
It's also a little strong as a comparison, people do plenty of immoral and 'bad' things all the time and there's no outcry to make then illegal. Should cheating on your girlfriend be illegal?
I'll say again, if I could tell every ISP out there how to advertise I would not allow the use of the word unlimited as it's currently used. However, I can't do that, the ASA can't find legal grounds to do so and being in competition it seems unlikely every ISP is going to enter a gentlemen's agreement about advertising.
So what's left? Not a lot except for consumers to wise up, which will do them no harm. It's also a minority of consumers, if you cap at 100GB or even 50GB then you're still affecting less than 10% of the customer base (see my figures on the first page). The other 90% would never be affected by capping and so it hardly matter whether they understand or not right now. Virgin's different caps at different times is harder to justify but all the information is out there.
If there's one legal provision which would be useful it'd be requiring an ISP to tell the user when they were being throttled or capped, just by email would do, but a brief note to say 'you've downloaded far too much, you'll be shaped to 512k for the rest of the calendar month'