Wirelessly posted (BlackBerry: BlackBerry8530/5.0.0.1030 Profile/MIDP-2.1 Configuration/CLDC-1.1 VendorID/417)
"Also why should Avis only charge for some airlines?"
Because the U.S. government taxes Avis on miles credited to domestic airline programs.
As to why Avis charges more for some domestic programs than others, that I do not know.