How stupid are you. They aren't saying you can't retire, they are saying you can't retire through the program.
My guess is it's either to remove the incentive from people that management actually wants to keep around or deny it so they can get more people in the involuntary program. From a headcount reduction metric, near retirement eligible employees will likely go in the next few years anyway while someone in the 40s has decades to go. The voluntary program is essentially a PR stunt.