He advertised it. He told her, and others, that he was going to do it. And he did it. No one stopped him? Not one pulled him aside and said something as simple as, "Hey sir, the kinds of things you're saying... they're making X uncomfortable and I think you're going to get in trouble. Maybe we don't do that at her promotion."
It's possible that the people he told about planning to motorboat the female soldier didn't think he was serious. It reminds me of the guy who used a big wooden mallet to
"pound in" the rank of the soldier in the 82nd, which resulted in a debilitating injury. Yes, there's a video, I'm pretty sure it's posted somewhere here on the site.
Everyone saw him wind up with it. No one thought he was really going to do it. They thought it was a joke. They thought it was funny. Ah yes, the ever-funny "hazing" joke. No one was laughing when the soldier got hit so hard he went into a seizure.
How about we keep promotions about the soldier, and don't try to make it about us with shenanigans and hazing? That would be great...