The most likely reason is that the right's holders to the programme want to be sure that you can't grab what you are watching from your tablet because of unauthorised changes to its OS. So VM have to prevent jailbroken tablets etc from accessing TV Anywhere.
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There are several things that can explain this move. Now, bear in mind that I don't fancy DRM mechanisms at all, as I find them particularly counterproductive.
It think it's simply to avoid the possibility of recording the stream on your iPad, to then make copies that could end up on the Internet.
Another thing would be to avoid mirroring the screen using AirPlay (or anything else that could allow you to get the picture on a TV), as to try to get you to rent another box if you were to want to watch it on another TV.
Now, I agree that chances of someone doing that are very limited, plus I would imagine that there would be some other ways of doing these things anyway, but that's the way it is at the moment.