If they don't help improve the customer's ad campaign they run the risk of the customer actually cutting their AdWords budget if sales decline as a result of a bad campaign.
So their goal and yours are pretty much the same, making more money.
I know this from personal experience selling direct ads to customers on one of my sites because the more money I help the customer make, the more ad budget the customer will spend on my site. Likewise, if the customer isn't getting the ROI, they pull the ad campaign and I make squat. Therefore, the customer and I have the same basic goal to make the customer ad campaign as lucrative as possible and that's not a conflict of interest.