In the free Windows software, I usually find that the author has inserted code in his program to run services in the background without my knowledge. For example, I downloaded a Windows program to un-zip files. I later found that this program had been running another program to track my movement on the internet and relay that information back to its author. While the same thing could be done under Linux, it is much less likely to succeed on a properly configured Linux system. Most programmers who write free software for Linux don't insert malicious code into their programs to begin with.
Problems can still arise when installing new programs, though.
Linux is such a stable operating system that users often to not do an entire system upgrade every few months. They simply upgrade whichever programs have bugs, but leave the rest alone. However, to force the Open Source and Free Software communities to use the same kinds of warranties would be a mistake. Many of the programmers in these movements are volunteers from around the world, so there is a jurisdiction issue. Aside from that, though, programmers in these movements are responsible enough not to have such behavioral enforcement.