I first discovered Sripraphai, New York's first truly authentic Thai restaurant,a bit over ten years ago, and I was just about the only non-Thai customer. Now back then, some Thai restaurants in Manhattan would write the customer's apparent ethnicity on the order, and the kitchen would adjust the spice accordingly. Caucasians got bland food, Thais got the truly authentic stuff, people who looked Asian but were not Thai got food almost (but not quite) as spicy as Thai. Sripraphai gave everyone the same food, but I didn't know it, and I would always ask for "spicy, spicy SPICY!!!!" One day the manager came over and said, "excuse me, you've ordered a dish that ISN'T spicy and we know you like spicy food, is that okay?"
Of course it was okay. Some Thai dishes are designed not to be spicy at all. Each dish is a harmonious creation, a perfect, symphonic blend of flavors, and I'd never tamper with greatness, just as I'd never tell Picasso, "Hey, can you paint something with less blue, maybe a bit more red?" I want it prepared the way it was designed. But how can I explain that to a waiter... especially given my limited Chinese and nonexistent Thai?
A similar problem: Thankfully, I can tolerate huge amounts of capsicain. But what about someone who can't? If he asks for food that isn't spicy, they will believe he doesn't want any spice at all, and will give him something totally bland and flavorless.