The best talent are left-brain thinkers; the best sales reps are right-brainers. The left-brainers are the creative types--the ones who simply dazzle on the air; the right brainers are the business-savvy folks who dazzle clients and your bottom line. So, (Seinfeld reference ahead) when the inevitable relationship between talent and client is formed, it can turn into a "bizarre-o" world of strained relations, broken promises, and unmet expectations.
Air talent by nature constantly seek positive attention--that's what makes them so good at what they do--but this need to receive positive feedback from those whom you have a business relationship with can be downright dangerous to the bottom line. I see talent promise "extra mentions," work side deals for endorsements, and interfere with business decisions all the time--and they don't even realize it.
It's not malicious, and it's not intended to negotiate against the radio station. On the contrary, the talent usually believes that they are offering "added value" to the relationship. As a GM or GSM, you must watch these relationships like a hawk. Speak to your talent and engage them. Teach them about the business of radio (most really don't understand business) and educate your reps about the psychology of talent. Your talent is one of your most important assets. Educate them to work with sales and clients. I think you'll be pleased with the results.