I didn't realise that airlines charge other airlines for lounge access (though it does make sense). I'm surprised they do this as I've often been into one lounge (say Cathay), found it too busy and gone into another (say Qantas) within minutes. If Cathay then charge BA for me and my guest I feel bad given I did nothing that cost Cathay!

Granted this is only applicable really at T3 for me. I'm just surprised they don't have a policy of "choose a lounge and stick with it" or "if the airline you are flying has a lounge you use it".