Why do newspapers allow scam artists to post ads?

Every time I get my town or school paper, or even Thrifty NIckel, they are filled with scams. Envelope stuffers, jewelry makers, online jobs that guarantee millions, and countless other ads that look suspicious. Is there any responsibility on the part of these papers to protect their reading public? Don't they know they are just helping these people take advantage of others? I know newspapers are desperate for revenue and the public is desperate for jobs. I know that people are gullible and should take some personal responsibility, but shouldn't newspapers take personal responsibility as well?