Because a lot of honest people just may choose to not use it if it costs. For many it's a convenience, but not enough to pay by the MB for. And if they do need to use it and will be charged it's good Idea to know so they can properly manage the usage.

It's no different than not knowing if Text Messages are included in a Plan before you use that feature.

Oh should mention I used USBModem and ICS (not long) and later ported that # to AT&T which forced the account closed with an ending balance and I don't see any sign of data charges. YMMV.