This is total conjecture on my part, but part of this mentality might lie in the fact that Craigslist is user-moderated. You shouldn't be circumventing the duplicate, spam, and misleading entries (which helps only you) you should be seeing and flagging them for removal (which helps everyone).
I like to think that Craigslist isn't free, I just pay for it with my time and attention by flagging the junk that doesn't belong there.
That's an interesting point but that still doesn't explain why he would be against someone else doing it another way.
Craig very famously types the command "sweep the leg" (a Karate Kid reference) to remove spam. Maybe it's the same command for small projects based on the service. :-)
I like to think that Craigslist isn't free, I just pay for it with my time and attention by flagging the junk that doesn't belong there.