They don't, but they do. Well, at least the Navy doesn't/does. Meaning it's against the rules, but people still do them for special occasions. Blanket party = you throw a blanket on someone and beat the shiz out of him.
im in the air force and i went through boot camp about a year and a half ago, and we had blanket parties all the time it's where we get someone in the middle of the night, cover them in a blanket and beat their ass. most of the time it was the bitch ass kid no one likes. we did all kinds of shit like that