The U.S never banned crypto. Binance had to block US residents from accessing their site because they didn't have any license to operate there.
I doubt their activities warranted money transmitter licenses, but they've been skirting their obligations under the federal Bank Secrecy Act and should have registered with FinCEN as a money services business.
My suspicion is that
FinCEN's May 2019 guidance -- which suggested Binance was operating as an unregistered MSB -- is the reason they left the US market. The guidance destroyed any ambiguity about Binance's obligations, which offered them the chance to go "Oops, we didn't know!" and rectify the problem by leaving the market. To support this narrative, they immediately registered a new entity with FinCEN.
Binance was playing dumb. We'll see if it works out for them.