One major effect of this is that weed stores will be able to use banks and payment processors legally once the regulators catch up.
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It's not just "You can buy with cash, and we conveniently have an ATM available to get cash if you didn't go to your bank."
I have heard from a very reliable source that the ATMs in most weed shops on the Eastside of Seattle dispense cash because you're going to be required to pay with cash at the counter. There are allegedly a few exceptions, but the majority of shops accept only cash and the ATM dispenses bills.