I'm in the same boat. I cannot tolerate dairy products and feel like I live in a cheese-crazed world. I agree with you and the OP that it's unreasonable to expect substitutions or a custom meal for $6. I have a different rant - I wish more restaurants would list ALL the ingredients in a dish. Too many times I have felt false confidence only to get a meal with cheese or a creamy sauce on top that was never mentioned in the menu description even though the other 8 ingredients were. I realize it's up to me to make my restrictions known but should it always be necessary?

I just got off a 7 day cruise. My first meal on board was a "barbecue burger". All the other burgers identified cheese as an ingredient. This one did not. Guess what came on the burger? One night in the MDR, I explained that I could not eat dairy products to the server. She went to the kitchen and came back to tell me that my only choice on the menu was the salmon and it would come without the green beans. At least there was something I could eat.

I have no idea what this family's issue was. But they probably were in the wrong place to be so particular.