Why? The NFL is a business. If the Saints are making him a competitive offer and he sees that as his best chance to compete for a Super Bowl, why not?
Put yourself in Cam's shoes. The Panthers are under new ownership and are almost certainly soon to be an entirely different organization than the one he's played his entire career for. This new owner and organization would basically be throwing you out with the old guard. If I was him I'd jump all over the opportunity to go to the best division rival and hand them their asses. I'd probably do it even if I had better offers on the table elsewhere to be honest.