It did cross my mind that getting the homeowner to pay the fine/bill might be a problem.  So if the rules of the HOA cannot be enforced what is the purpose of a HOA?  I am asking that as a sincere question not as a slam to the HOA.

I'm sure they're getting notices that you just don't know about. 

If nothing else the HOA keeps the common area nice and picked up.

 As my husband says, rules, laws, etc simply keep honest people honest and unfortunately won't do much for the dishonest.  He kind of has a point, the people who are going to break the rules will do it regardless, the ones who are going to follow them, having them in writing just makes sure they stay that way.

The HOA can and does fine people for violations.  If the owner refueses to pay the fines, the HOA can take them to civil court....but it usually takes awhile because it cost alot of money for the HOA to do that.  It has to be worth the HOA's time and money to go to court.  As far as what the HOA is for....well, the HOA manages and pays for the common area keep up, power for the street lights (not payed for by the county), pays for the water, etc.   If you look at the financials online, you will see what the HOA pays for and what they do.  Managing weeds and other violations is the smallest part of the HOA.