Does Starbucks use WPA2? The last one I was at was open, I just had to click through a terms of service page.
The whole standard it a mess; it should have opportunistic encryption on open networks, then clients can display a warning if this doesn't happen for whatever reason ("Anything you send or receive over this network may be readable by others" or similar).
If you are sniffing the 802.11 frames (and you should assume someone is) and you catch the entire 4-way handshake and the nonce generation is predictable you could reverse-engineer it, but then again you can say the same thing about a TLS connection too.