I guess I missed the "even worse" part.
Yes it would have been nice if the manager had sent the old homophobes packing - and no doubt that's what would have happened in my neighborhood, where nearly all of the restaurants are mom-and-pop places. But this happened in a "well-known chain Italian restaurant" (Olive Garden?) and so the manager is ultimately answerable to the corporation. If the couple had been ejected, they'd have complained to corporate HQ, and the manager would have been disciplined. In the world of franchise and chain businesses, it's always better to inconvenience your employees than your customers. Keeping complaints from reaching corporate is Job Number One.
As it happened, the manager behaved correctly - he went over and gave the couple a "stern talking to". But then he chose to let them stay, which irked the offended server, who further complained that they were allowed to remain in his section - "in full sight of me". This just sounds like histrionics.
And the story had a happy ending for the server because a regular customer who witnessed the incident showed her support on the spot, and later complained to the restaurant manager as well as managers two levels above him.
So, now there's a policy in place to prevent this from happening again - because the supportive customer was the one who complained. If the homophobe couple had been ejected and complained, a very different policy could have been established.