Crosman has entered into a “strategic licensing agreement with the U.S. Marine Corps for the design, manufacture, distribution and marketing of U.S. Marine Corps branded airsoft products.” (marketing speak for licensing/endorsement deal). Crosman is no stranger to licensing popular brand names. Remington and Marlin branded air guns are, for example, actually made by Crosman.
Airsoft is a popular sport/game played by potential USMC recruits (teenage boys). It makes perfect sense for the USMC to put their name and insignia on airsoft guns.