What I don't get is this: where do they get their authority from?
Like a 'real' government, the consent of the governed. In the case of an HOA, it's generally a non-profit corporation created at the time a housing development is built - often by the developer. When the housing development is finally sold out, the new residents take over the corporation. That's why you seldom see an HOA in rural areas or in towns that 'grew up' without suburbs or intentionally-developed areas.
The theory is that the HOA gets it's authority from the members of the housing area; they agree to be members of the HOA when they buy a house in that development. They don't have a choice, it's a deed restriction. So either they agree to be members of the HOA, or they cannot complete the sale.
That also gives the HOA the power to levy fines, enforce regulations, collect fees, and in extreme cases, seize houses and sell them without the owner's permission.
If I buy a plot of land, it is mine to use, and the only things that apply are the local zoning laws. If I buy a house somewhere, it is equally mine, and any contracts signed by the previous owner are null and void.
Actually, that's generally not true at all. In addition to things like the mineral rights or other rights that previous owners may have sold (meaning they cannot convey those rights to you when you buy the property), there are deed restrictions to consider. This is why it is so complicated and expensive to close a real estate deal in the USA; for one reason, a title search has to be done, and any deed restrictions discovered and disclosed.
http://en.wikipedia.org/wiki/Restrictive_covenant
A lot of people don't know about them; they assume that when they buy a piece of land, it is theirs to do with as they please (other than zoning laws, as you stated). But no...
And you don't have to live in an area that has an HOA to have a restrictive covenant on the land. Some people have been surprised to find that they bought empty fields that they intended to develop, only to find out that they were forbidden from ever building on the property by the deed itself. Such things can be fought in court, but it's expensive, time-consuming, and if you lose, you own land that you can do nothing with - literally.
Check THIS one out!
http://savannahnow.com/stories/020698/CMNstock.html
The weird thing about deed restrictions - you can be the second, third, or one hundredth owner of the property - only the person who put the restriction on the deed can remove it. If that person is deceased, then it cannot be removed, as I understand it.
It seems counter intuitive - if you own something and sell it, you sell all interest in it. But not land, not in the USA. You can put your stamp on a deed by placing a restriction on it, and that restriction will have to be obeyed in perpetuity unless overcome in a court of law.
I could understand that IF you sign the contract, then the HOA has you by the shorts and curlies. But signing the contract cannot be (in Belgium at least) a legal requirement of the sale.
In the USA, it can indeed be a legal requirement of the sale. You agree or you cannot buy the property.
So what am I missing here?
Real estate law is weird and complex in the USA. Nothing a person could guess, and most Americans have no idea about it either.