Does the money add up for that to be a deterrent? Are those using them for general VPN access paying enough that they'd care much, and if so is that enough compared to what they'd lose by kicking off the users abusing external sites (they can't all be using just free plans, and anyway tests to kick them could have enough false positives to lose them good paying customers as well as bad users). In any case is the amount of traffic enough for them to care to spend the effort tracking it to identify the bad users?