True, it is just steel stock, bent and punched. I guess the question is, what grade of steel? Grade 1 might be too soft and shear too easily. Grade 8 might be too brittle and risk snapping. I dunno. If you know what grade of steel they use for hitch pins, you could surely match it up in a bolt, and be perfectly safe.
However, I guess the issue might be that a bolt is not approved for use as a hitch pin. In the case of a problem, if you are using the "wrong" stuff, you become liable through negligence. If you use the "right" stuff, the manufacturer shoulders the liability in case of failure.
I use R-clips for local short-term towing, and a brass pin-lock for non-local or longer-term towing, and remove the stinger when I'm not going to be towing. I lube the lock once a year or so, and have never had problems with it binding up. Years ago, I left a stinger in over a winter, and never did get it back out, even after beating the snot out of it with a mallet. Now I take it out if I won't be towing again within the week.