Here's a wild guess why (and I am not saying this is logical):

I understand that the "no arriving pax" rule is to protect HM Customs' revenue. Here's the evil it is supposed to prevent. Maybe the RCC and other lounges get their alcohol duty-free or on some tax-favored basis. If arriving pax could drink freely at the RCC then they would be importing alcohol beyond their allowance when they eventually slither through customs. Or maybe since the drinks are free, UK laws give the lounges some exemption from licensing (or licencing) laws because they serve only people who are leaving the country immediately. If they served arriving pax then they might have to qualify as regular public houses.

Neither one makes much sense to me, but my own delightful home state and country have many laws which seem as silly.