I guess ordering dishes out of your own Android/iPhone feels more like ordering a take-out instead of a sit-in? I think e la carte is intended to offer more on enhanced dining experience, rather than just an easy way to pay for your dinner.
One possible argument for proprietary tablets is that the value to a potential thief is much less than a commodity tablet which could be repurposed. Perhaps the maker of a "standard" Android tablet could be convinced produce a version slightly modified so that their standard distro would not work without modification? While thieves clearly could hack the standard distro to make it work, would they really achieve high enough theft numbers to justify the effort?
I think buying general purpose tablet would definitely cost much more. Also, pushing restaurants to first buy an ipad/android tab, and then using your app on it is just too many hoops to jump through for any restaurant.
