I think airlines are very, very good at revenue management. They realise that as soon as they create a nice new lounge, every single FF, from every single alliance, will start using it *unless* they have restrictions stating that you have to be travelling on their airline (or an alliance airline).
Otherwise, every single Star Alliance Gold, Skyteam Elite Plus would be using the new QANTAS first class lounges in Sydney and Melbourne!
I'm pretty sure the poster was implying that WP's and OW Elites would access QF and OW lounges when flying other airlines, not *A Elites visiting OW lounges.
I don't blame them for imposing their rules. It's their lounge, they can admit who they wish