The iPhone begs for wi-fi regularly (especially if you're looking at, like, maps, or downloading things), makes it easier to connect (popup captive portal dialog technology, Wispr) and makes it more tedious to turn off. Android phones make it easy to turn on and off (custom widgets on the home screen) but do not make it quite as easy to connect. Therefore you see a lot more iPhones hanging around on random free wifi than Android, in terms of installed-base percentage.
AT&T's early iPhone monopoly and the (in)ability of their network to handle the increase of traffic in those early days is probably a factor which contributed to this state of technology.
Not only that, but iPhones will automatically connect to any open ATT wireless nodes, they are configured that way when you go purchase a device from ATT.
AT&T's early iPhone monopoly and the (in)ability of their network to handle the increase of traffic in those early days is probably a factor which contributed to this state of technology.