That story was so incredibly poorly reported.
The real story: The folks who make Mad Men always intended for it run a limited number of seasons. And they are the ones, saying now, that they won't want to be making Mad Men beyond Season 6. (Actually, the network is upset about this... they don't want to plan a specific ending date for a show, at this time.)
This is no different from what they're doing with Lost, deciding to end it this year. Lost isn't being "cancelled" (or "axed", which is a word I saw used in reference to Mad Men, in an article earlier today). The producers are deciding when it should end. That's actually the ideal situation for a long-running serial drama.