My personal guess is, they probably want to capture the market for mobile web connection on tablets, and offer cheaper packages for that. But since they don't want you to circumvent regular mobile calls on your phone and use voip instead, you can't purchase these plans with mobile functionality. With phones or dialer-equipped tablets, you'll only get pricier traffic plans. As a bonus, if you want to have mobile calls/sms and cheaper web, you'll have to get two contracts—what's not to like?