How would Disney corporate know

Of course they wouldn't. But requiring ticket quotas on their films de-incentivizes theaters from offering comps to their staff, which is a reasonably longstanding tradition.

Also the issue with 1 person serving 60 people sounds like a theatre issue

It isn't. Requiring ticket quotas on their films asks theater owners to throw away their knowledge of their customers and focus on getting butts in seats for Disney films. You don't just schedule a second staffer because you hope to sell more tickets—second staffers cost a lot of money. Most theaters, which are chain operations, are going to add labor as a last resort. Means that in the meantime, the existing staff is getting held in the fire. Adding food service to a theater is wildly expensive, and OP is describing a practice that hamstrings how a business can delegate that investment, i.e. if you're serving food in your best theater the week Black Panther comes out, you're gonna be serving food during Black Panther.

Those are not Disney's fault.

Wouldn't theaters already be putting Disney films on their best screens? Sure. But maybe not weeks after release. Disney's controlling how theaters can screen their films—and affecting theater staff correspondingly—a month after the fact. Management has to agree to that to screen Black Panther. Management has to screen Black Panther to stay in business. If management loses money due to having had to make Black Panther its only priority regardless of its local market, the employees feel it first. This is absolutely Disney's fault and it makes great business sense. It's just anti-consumer and it blows for audiences and theater employees.

Here's Tarantino talking about Disney screwing him with these very tactics a couple of years ago.