They won't let you punch in an expired prescription, let alone "whatever you want," but if you've bought a pair of glasses from them in the past, they'll let you buy another pair even if the prescription is now expired.

IIRC they basically try to chase up your eye doctor for you, and if your eye doctor doesn't respond within a certain timeframe, they have a greenlight to proceed with the sale according to a California law that was passed maybe 10 years ago. It's probably different in different states - maybe in some states you can literally just type in whatever you want.