If you think about it, £70 equates to around a dozen or so CD albums, and probably not even that.
One of my friends has downloaded thousands of tracks from CwM within her first few months and she hasn't had any such threat of having her service cut off.
I think in the T's and C's it says that they send you a letter if they think you are abusing the service (which is supposedly unlimited!) and warn you that they will cut you off if your habits continue. I have no idea how much downloading you would have to do to warrant one of these though.
You might be able to find a pay monthly unlimited service which won't have DRM built in.