Well... you're absolutely right! I'm on my second project right now, and would never dream of guilt-tripping a client into a sale :-) But still, a lot of successful projects get started without much regard for ethics. This especially applies to online communities: LessWrong's launch is actually an outlier. Creating hundreds of sockpuppet accounts to simulate active life on the site is pretty much standard industry practice, Myspace got its startup push from a huge spam emailing (insider info I saw somewhere), etc. Because the choice is either this or 5 visitors/day, month after month, who look at your comatose website and leave.

For an especially clear-cut example, SEO is certainly unethical from the customer's point of view, but I absolutely have to do it, and will. Sounds a bit like PUA practices, no?