Its simply due diligence. Most if not every GM is probably and has probably called on Rondo at one point in time or another over the last 4 years. My guess is the calls go something like this:
Danny: Hello
Other GM: Het Danny! Its _______________
Danny: Oh hey, what's up?
Other GM: Is Rondo available.
Danny: Depends. Are you offering me just one player that is better than Rondo?
Other GM: No. I just have a couple guys who's salary matches Rondo's.
Danny: Then no. He's not available.
Other GM: Okay, thanks,