You're not being dense. Its inexplicable. The only thing I can come up with is that "5" looks like "S", so maybe its banning "MPS", but even that is nearly mean

You're not being dense. Its inexplicable. The only thing I can come up with is that "5" looks like "S", so maybe its banning "MPS", but even that is nearly meaningless; urbandictionary has some explicit things it stands for, though they're not well-upvoted.