About the towing issue, I own a small towing company in Salt Lake and usually the HOA has to be the ones to have it towed away. It is called a "private property tow". Certain legal rules apply, as to immediately reporting the tow to the police agency, and to the tax commission/UDOT, sending certified letters to owners, 2nd owners, lien holder within 2 days, etc.
They probably use a particular company who has the agreement with them to do the tows. And yes, you may see their signs around the parking lots or at the entrances. How would non residents know the rules changed?
Usually the owner nor tenant can call a tow company of their choice to have it towed away. The tow company may patrol or only come when the HOA calls them.
HOA's will usually fine the owner for any problems caused by tenants. How can you prove your tenant isn't responsible for the person who owns the truck? Your tenant may not be telling you the truth. Demand the HOA tows the truck the next time it is there. It costs the HOA nothing. The truck owner is the one who will have to pay.
Also, for an HOA to change or make new rules, they have to go thru quite a procedure usually. It's very hard to get the number of votes they need to make some changes.
Do you have an updated copy of the docs and bylaws?
Sorry, this response is so fragmented. I've been told I have a problem with that! ;-)