One of the reasons, besides safety, to ban all mobile phone usage on board is that one of the main pleasures of flying is not having to listen to stupid loud conversations you are not interested in! An aircrat should not be like a bus!
Now a question: If a passenger is using their phones, say, after take off, and I (as a fellow paying passenger) take the damned thing from him and remove the batteries... Is the law going to be on my side? Am I liable for theft? or assault? or is it considered as an act of self defence, or safeguarding the general safety? Would it be regarded as a kind of "citizen's arrest"?