It is right there in the Patent Act. Patents are a form of personal property and are freely assignable. 35 U.S.C. §261. They may be bought & sold, licensed exclusively and otherwise.
Now, some patent groups (EFF) are up in arms because a bankrupt company, Commerce One has auctioned its patent portfolio that reportedly covers computerized methods of paying bills and obtaining supplies. Their objection: “the buyer is to require a substantial rate of return.”
Give me a break — what kind of objection is that? The underpinning of the American economy is based on the exchange of property and obtaining a return on investment.