My M&P Instruction manual says "+P ammo generates pressures in excess of the pressures associated with standard ammunition. Such pressures may affect the wear characteristics or exceed the margin of safety. Use of "Plus-P"ammunition may result in the need for more frequent service."
Their stand on +P+ says "Must not be used"

You can interpret those for what they are. I say they are just not endorsing +P use because of legality and wear and tear on their "Lifetime" warranty products, but it can shoot +P rounds.