The metaphor doesn't work. You wouldn't dangle a steak dinner as a reward to a group of people who you could reasonably assume were mostly vegan. The intended reward _is_ the steak, because steak is expensive and delicious when prepared well. Yes, you could order a nice salad, but don't you think it sort of misses the point? It's not on the employee to enjoy the reward they're being offered, it's on the management to pick a reward that's valued by the employee. And sometimes, that means having even a tiny bit of cultural sensitivity.
That's debatable, I'm American and would take an offer of a "steak dinner" to literally mean steak. Even presupposing you're correct that it's generally understood by Americans as an idiom, assuming that non-Americans understand American idioms is a form of ignorance, if not deliberate arrogance.
No one's saying "assume they are vegans," we're saying "don't assume they eat meat." Big difference.
I'm American and if someone offered me "a steak dinner" I would expect to be taken to a steakhouse, where steak was pretty much the only item on the menu.
I'm American, and if someone told me they're taking me out for a steak dinner, I'd immediately assume we were going to a steakhouse. It couldn't be any less of an idiom. If they took me out for pork chops, I would be completely and utterly confused.
