That's really a great point but Joel's board is not exactly the same as Craig's list. Joel's point was that Craig removed the revenue from classified ads, when the money was being used for the public good. He acknowledged that the classified ads market was fair game, but he disagrees with Craig's view that he's doing the most good keeping it free.
I think there's a notable difference to Craig's list being free vs Joel's job board costing hundreds of dollars to post. ($400 iirc) With Joel's board he's inviting competitors to under cut him, and there's still a thriving job market outside of his control. Craig's list on the other hand could be argued that by keeping the cost to zero he's making it difficult for anyone to make money off of classified ads because there's always a place to go for free ads. And if nobody else can make money at it then they can't have the option to fund Investigative Journalism and democracy and all that.
I do fail to see why any other industry couldn't fund reporting, but classified ads were a good fit.
Now I really enjoyed the argument he made, but I think Joel should put his money where his mouth is. Joel never argued that Craig should fund reporting as a good business decision, he argued that if Craig wants to serve the greater good (which Craig argues he does) funding reporting is a better way to do it. I'd like to see Joel take a % of his job board and fund the greater good. Part of the problem is how does one help fund investigative reporting? I'd say invest in a newspaper but they might not be the best source. He should run some experiments, hire an investigative blogger, start a grant program for reporter interns, stuff like that. It's his money, but this is obviously a topic he feels strongly about.