i can tell you that here in germany at deutsche telekom, kabeldeutschlan/vodafone and unitymedia it is indeed seperated from your account. there is some separate "secure" channel that i have no access to unless i were to hack that wifi router.
in any case, i don't think it's a big issue. customers actually want this kind of functionality sometimes and most that give out a share of their bandwidth indeed never notice. i can tell you that it works well enough in practice. normally the bigger problem is to even have a working base service. if using your wifi AP as a cash income may motivate them to keep your DSL line operational that's actually a plus for me
. Cause stuff is often unstable.
Also my DSL modem for example could negotiate a higher modulation with my DSLAM, if they decide connect me with higher speeds because I bring them more customers I would theoretically benefit cause they have more incentive in making my connection fast and cost-efficient
as i said, this is just a business model issue, you're just outraged cause you didn't read the fineprint of your contract properly. but if they had asked you before and you'd have gotten a share of the revenue they make via your router perhaps you would have even agreed. especially when they say that instead of 16Mbit you get 100Mbit with 16Mbit guaranteed. As people don't care and just pay them nothing will change though.
But if you're really paranoid there's an easy solution for you (who clearly is one of the few who cares): open the device and short-circuit your antenna connector by putting a bunch of solder on it. this way you don't have to understand the technology behind and still be 100% sure that nobody can share your wifi.
Then attach your own wi-fi router at the ethernet port.