Hmm, the same reason you wanna keep your conversations private?

I can think of few cases, like if you want to giving nearby people coupons, I mean really nearby, not random guy on the other side of the planet trying to re-sell your coupons.

How to use a non-hackish, non-intrusive way to make content exclusive to local users?

Providing an ad-hoc wifi is not the solution because people may need public Internet activities and a device can connect only to one access point at a time.