Should all housemates be expected to contribute equally (if at all) to the TV licence fee?

Yes, they should all contribute. The payment is for it to be there when you want it, not how much you use it. That's similar to what I was told when I said I felt guilty playing computer games on days I had nothing to do. My supervisor said I was hired for my knowledge and ability to do what was necessary when the need arises. So you pay for the TV to be available, not necessarily used 24/7.