Although I consider the practice unethical, the other side would (does) argue: "Who cares? You are trying to get 20,000 customers and you pissed off 100 to get there."

I still can't bring myself to do it, though.

You wouldn't really have any customers if you weren't accepting payments though, it would be more like you pissed off all of your potential customers just for a bit of market data.

This is advice Tim Ferriss actually gives in his first book. I generally agree with your sentiment.