Most authors are savvy enough to retain their own copyright; AIAA would love to own it, but does not require such in order to present a paper. In the case of government funded work, such as NASA, the copyright cannot be exclusive. AIAA will still sell the paper for fifteen or twenty bucks, but it should be available at no cost through other, government sponsored sources.
Some university originated work will be made available by the authors, particularly if it was generated through a government grant. It depends on who takes the time to create the necessary website, etc.
Corporately sponsored work, such as that from the folks employed by the manufacturers, will of course be available only through AIAA.