Perhaps craigslist could put up a checkbox (like they do with the never-checked "It's okay to contact me about products ...") that says something like:

"I'm okay with people finding this listing through another service."

Perhaps because they don't want people to get to their listings that way even if they want to, and thus don't want your opinion? (Edit: and perhaps they really aren't as interested as they claim in making it easier for buyers and sellers to find each other?)