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I had it when I was buying legit stuff from a seller who also seemed to sell stuff that was restricted in the US-payment gateways normally get a blacklist they have to comply with to prevent money being sent to them. I think Coinbase also banned me because of this...
Could also just be automatically flagged by some kind of software as a pattern of spend rather than the amount.
I'm sure it's in their terms and conditions anyway...
Fuck the US government.