Telegram does the same. They just revoke your API access and on continuous violations ban your and your users accounts. Currently there are almost no apps daring to cross that line.
Contractually?
It's far worse option than directly taking money for the API access.
Depends, if your goal is to increase the value of your ad sells not necessarily.
But the point is really that there are plenty of options, and instead of attempting any of those Cs just went with api euthanasia.