Really? How many websites are signed up / do you expect to sign up? Can you not spare 5 minutes per site per month to make sure there's a notification and/or opt-in? Or come up with an automated way to check it. Or use mechanical turk and pay somebody $0.50 per site to check for you.
If you can't prioritize something as important as running an ethical (and law abiding - take a very close look at the ramifications of unauthorized computer usage, which I think it could be argued you're doing with this platform) business, then you really shouldn't be in business.
If you can't prioritize something as important as running an ethical (and law abiding - take a very close look at the ramifications of unauthorized computer usage, which I think it could be argued you're doing with this platform) business, then you really shouldn't be in business.