I'm with you on demanding an account. However, if you have self-serve chargers that you want to be available 24/7 while also being unmanned, accepting cash can be troublesome. I hate trying to get cash to be accepted by automated machines, as they have to have the bill in the perfect condition or they just spit it back out at you. Also, an unmanned kiosk accepting cash is just asking for security issues. Yes, it's just a reverse ATM, but if you can avoid it by not storing cash then so much the better for them.
Vending machines do just fine.