Dirty Fish??

Can anyone help me understand why people have an aversion to tilapia? To me, it's just a regular white fish with a mild flavor. I've eaten it tons of times at restaurants and at home. At my restaurant, we have a Panko-breaded and fried fish dish in a bed of red curry sauce. It is absolutely delicious and our best seller. At dinner (it's more upscale/casual) we use snapper, but at lunch (way more affordable low-priced lunch specials) we use tilapia. It's also half the price of the snapper.

So many times when people ask what kind of fish it is at lunch, I say tilapia and they get a grossed out look on their face and say "never mind" or "tilapia is a dirty fish". Okay? I don't understand this logic mainly because of lobster, which is just a bottom-feeding sea bug, yet is somehow expensive and fancy to eat.

Recently I had the man order the curry fish, and did not ask me what sort of fish it was until I was setting the plate in front of him. When I told him tilapia, he said he had to decline the meal. He still wanted to pay for it, wouldn't order anything else to eat, and tipped me 25%. (What the hell?!)

Is this behavior as silly as I think, or is there a real, good reason why tilapia is a bad fish to eat?