So I did some research....turns out that the term "pipe hitter" is a regional term common in vaguely north eastern America. It originated around guys who were the "muscle" for people who needed "muscle." Gangsters, hustlers, union guys, and the like. They were called "pipe hitters" because they would literally hit you with a piece of steel pipe. This is where the departure happens. Supposedly the term somehow or the other transmogrified during our latest forays into the Middle East (particularly OIF & OEF) to apply to SF and .mil guys who were "tough" and "got the job done." It is intended somehow or the other to be a pun on the "Pipe Fitters Union." Either way, there's an interesting thread
here. ARFCOM warning.
I've always heard the term in referring to addicts and fiends who were "hitting the pipe." So for me, it still seems silly but, to each his own....of course, I'm not nearly as operator as some.