When I've gone to America's Best for an eye exam, yeah, they were none too happy when I wouldn't also buy glasses from them, but I got my prescriptions to go. Fuck em. Didn't have a single problem with the optometrist office in the Target. I'm also pretty sure that the optometry part and the retail frames and lenses part of these stores are at least somewhat separate from each other, business-wise.
The down side of the online "cheap glasses" places is that when your frames show up all bent and twisted, you have to adjust them yourself, and if there's a problem with the lenses, that's a whole thing. Buying from a storefront, they'll handle all that for you. I'm capable of running my plastic frames under hot water to straighten them out and adjust them to my crooked head.