Perhaps also to block access to employees/crew/"buddy pass" travelers who have to travel standby....

Given that these lounges are not airline-operated, and that the front desk staff are not going to be familiar with the intricacies of each and every airline's policies for standby, employee travel, etc., my guess is they just decided to have a rule across the board that only confirmed travel with confirmed seat assignments (on any airline) would allow access. Simple, straightforward, not subject to interpretation.

But I agree that it is does cause a problem for last-minute purchasers. (If you are going to stand by for an earlier flight, make sure you keep the original confirmed boarding pass with seat assignment to show at the lounge entrance!)