The problem with this from a usability perspective is that in many markets, tablets ship without dialer functionality, and carriers frequently will block tablet ESNs from dialing and MMS/SMS functions. Running a VoIP client is always an option, but an extra layer of complexity that the target audience here is likely to struggle with, especially when it comes to troubleshooting.
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?
