If an advertiser contacts me and wishes to place an ad on one of my sites, I give them a quote. Adsense never even comes up in the conversation.
I do NOT check to see who is advertising on my sites via Adsense and then contact them at all, ever. In fact, I've never gone out looking for advertisers. They always find me. How they do it is their business.
I believe Google does not want their partner publishers bugging their AdWords users about ad problems, etc. I also believe Google does not want publishers contacting advertisers whose AdWords ads are showing on said site, to try and set something up "on the side".