As critically acclaimed as they are, AMC shows don’t make a lot of money. Apparently Breaking Bad currently breaks even and Mad Men operates at a loss. The only profitable major show on AMC currently is The Walking Dead (which is also the only show they own outright).
Because they do not own Mad Men and Breaking Bad, it means they don’t see any of the overseas licensing and DVD money.
This is (reportedly) why AMC has gained a bit of a reputation in Hollywood for not being easy to work with – they’re pissed at reaping very little reward from the shows they took a risk on and have subsequently gone on to win numerous awards.
It’s also (allegedly) why AMC and Darabont had a falling out leading to him being fired. The Walking Dead’s season 2 budget was heavily squeezed (remember all those very similar episodes on the farm?) because AMC wanted to maximize the profits of the only show they own.