I'd guess it has more to do with not wanting to give away free wifi to people who've jailbroken and unlocked their phones for use on other carriers.

BTW, the prompt you're getting is what I've been seeing at Starbuck's in Cleveland since the change was announced. It suits me fine since I subscribe to AT&T DSL service.

I think that businesses have the right to offer free WiFi if they choose. Many coffee shops offer here in Canada, and so do many other places. If I can sit and have a coffee and use free WiFi, all the better. If Starbucks doesn't want to lure people with this convenience that is their problem.
I prefer Wifi to some silly service tied to an iPhone. I think there is a fear that wireless internet will begin to erode cellular service and finally put an end to the ridiculous fees associated with cell providers. I myself, used Skype today to conduct a video conference half way around the world to Australia. The cost? Zero, its in the $50 a month I pay for DSL.
As new wifi technologies evolve, ranges increase and service becomes faster, I hope to see cellular go the way of the dinosaurs.

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Some people think what they write here is important. It isn't, and neither was that.

There's a well-known lesson here (continued).

Do your iPhones work at Panera? They offer free WiFi and thus are my preferred coffee vendor. Granted, they aren't on every street corner, but the cost of gas to get to one is generally a lot less than the cost of the WiFi service at Starbucks.