When you do the right thing and order spinach instead of potatoes and toast, there's something uniquely annoying about a waitress saying slightly sarcastically, "that's a really healthy meal".
The other thing that drives me _nuts_ is the stupid awkard interaction about the upcharge for a vegetable. If you say "Could I have spinach instead of potatoes and toast", you have to engage in this embarrasing, complex negotiation. I have resorted to just saying "Hold the potatoes and toast, and give me a side of spinach." Avoiding the pain point and the embarrasement of seeming like you are trying to get something for nothing is more important to me than the stupid $1 up charge.
It's funny how even a small amount of friction reinforces bad behavior.