Well first off we are talking EARLY 80's so tows, (in town), were more in the $30 - $50 range. Not today's rates.So with NO evidence what so ever that the guy was going to get back on the bike, after you explained to him that riding with no helmet was illegal... you just decide to make his already lousy day worse by adding a $300-500 towing bill.
Can you see why we don't like cops.
Now to address your other concern, about NO evidence. that he would ride without a helmet. If I had pulled him over for riding without a helmet then he has already demonstrated his inability to follow the law, and that would be my "evidence" that he may indeed, despite being told not to ride without one. Not everyone who is told to do something by a police officer, follows those instructions, so yes, I would tow.
Now do YOU see why officers don't always accept what they are told? lmao
Also if getting his helmet stolen was "his lousy day" dying while not wearing a helmet would have made that day a WHOLE lot worse.