This may be true on one level, but it is a little more complicated than that ...
AMC may want to keep Mad Men going but the reality is that it's not a massive cash cow like The Simpsons or Law & Order. It broadcasts 13 episodes a year and its cast, not least John Hamm, are doing an increasing amount of movie work. They will not be under contract to Mad Men forever and will at some point want to move on - and AMC won't be able to stop them.
It's also not in AMC's interest to be overly draconian with Matt Weiner. He's the person who created the show that's massively raised their profile and p*ssing him off would be a pretty stupid thing to do. It would send out a bad signal to the creative community, particularly those people AMC may want to wrestle away from HBO. The smart move would be to agree to let Mad Men end when he wants it, in return for a new project that can take its place.