Actually, the operators have no alternative to pay the MTR/Tunnels to provide coverage - unlike almost all other outdoor sites, where they could choose a building next door - there is competition. However, the MTR is impossible to cover from outside#, the MTR owns the antenna system which is technically complex and expensive to run (partially as a result of the MTRs own inefficient bureaucratic processes, 'cos they are a railway operator not a telco).
Originally the MTR negotiated a sizeable % fee based on the calls inside the MTR - remember it was $1.40/minute! Years pass, realizing the price per minute dropping and operators offering 1000's minutes for a fixed price (not to mention unlimited data for next to nothing) the MTR has wanted more $$$ and so must have negotiated a per subscriber top-up fee $18 with the operators (and probably OFCA/OFTA). The operating costs are more or less fixed to the MTR but their income is user/traffic dependent. Conversely normal outdoor sites typically cost a fixed monthly rent.
As for "I don't use it so why should I pay for it" - yes that is reasonable at first glance. Similarly, I don't use the sites north of, say, Boundary street - why should I pay for those? Perhaps those sites are actually more efficient and rent less, thus cost the operator less - so if my part of the network costs are not pooled with the more efficient, cheaper New Territories I might end up paying more.
Well it is the cost of doing business. They could say "OK then - we will block you in the MTR and that will cost you $50 extra to have MTR service blocking due to the extra inconvenience it causes to us and having to convince the MTR lawyers that you are prevented from using it in the MTR#" - then it might not seem like a great idea.
#You could probably still use it in the open air stations and sections of the Kwun Tong line and be happy that you are screwing over the MTR and avoiding the mobile operators $18 fee.