AT&T's MicroCell boosts cellphone reception by hooking into your broadband connection. To keep your monthly minute allotment from being used up, AT&T offers an unlimited voice plan for the MicroCell. They don't offer something like that for 3G data though.
An AT&T spokesperson gave this explanation after some folks complained about the lack of unlimited 3G data plans for the MicroCell—which does use your monthly data allotment:
The 3G MicroCell complements Wi-Fi by providing enhanced in-home voice coverage and reliable data when Wi-Fi may not be available — but it is primarily intended for voice calls.
Of course, this whole thing fits with AT&T's murder of unlimited data plans and all that. But really, if you're in your own home—with a broadband connection and Wi-Fi available—you shouldn't really be relying on 3G data through the MicroCell anyway. [Light Reading]