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"Hey if you like my stuff consider adding my <public bank account key> to your monthly payments....Banks specifically DON'T want you doing that because they don't want undesirable businesses funneling money in and out of their banks. I'm sure there are different legal liabilities for different places for different undesirable businesses with that kind of thing. It's probably a potential minefield to let anyone do that indiscriminately.
Probably have to track everyone, and then roll back or cancel accounts when you catch the pornographers, or cartels or what have you.
(Of course, that brings you back to the problem of randomly cancelled accounts.)