The reason for this is the distinction between copyright law and contract law. This is the law which the GPL relies on. If you obtain a GPLed program, you can use it all the live long day without being subject to the conditions of the GPL as long as you don't give a copy to someone else or make a derivative work based on the program.
Contract law is something completely different.
Contracts are governed in the United States by laws passed by the legislatures of each individual state. The UCC covers various areas of commerce, including contracts. One basic rule is that a contract can only be formed when the two parties exchange something of value. By definition, open source authors give their work away free of charge, so even though I use Samba , I don't have a contract with Andrew Tridgell, one of its authors. Say I go to the hardware store and ask to purchase a hammer.