I remember the old time techs telling me ATF is about equilivent to 5w fork oil which is what is recommended for the street in most jap bikes. It was easy to acquire and made service cheaper.
Honda used to ship bikes with a very light non corrosive oil in the forks (ATF and fork oil is hydrophillic like brake fuild - meaning it pulls moisture from the surronding air). This was a farily clear oil and was referred to as "fish oil" by the techs and eventually the hobbyists (because it smelled like fish). Part of the new bike prep was to drain this "fish oil" and replace with real fork oil or at least ATF. There have been tales of techs prepping bikes for sale and not draining the fish oil until the fork seals went. I'm sure there are a couple of bikes that went their entire service lives with super soft front ends. A couple of years ago I bought a set of forks from a guy that used to be a tech at mineola honda (one of the largest honda bike dealers BTW). He said they were used but low mileage and he had saved them as a backup for his own 75 cb750F. I cracked them open to change the fluid and sure enough - really really bad (I mean foul) smelling "fish oil".