Some time before 1989, Apple Computer, Inc. started a lawsuit against Hewlett-Packard and Microsoft, claiming they had breeched Apple's copyright on the look and feel of the Macintosh user interface. In December 1989, Xerox failed to sue Apple Computer, claiming that the software for Apple's Lisa computer and Macintosh Finder, both copyrighted in 1987, were derived from two Xerox programs: Smalltalk, developed in the mid-1970s and Star, copyrighted in 1981.
Apple wanted to stop people from writing any program that worked even vaguely like a Macintosh. If such look and feel lawsuits succeed they could put an end to free software that could substitute for commercial software.
In the weeks after the suit was filed, Usenet reverberated with condemnation for Apple. GNU supporters Richard Stallman, John Gilmore, and Paul Rubin decided to take action against Apple. Apple's reputation as a force for progress came from having made better computers; but The League for Programming Freedom believed that Apple wanted to make all non-Apple computers worse. They therefore campaigned to discourage people from using Apple products or working for Apple or any other company threatening similar obstructionist tactics (e.g. Lotus and Xerox).
Because of this boycott the Free Software Foundation for a long time didn't support Macintosh Unix in their software. In 1995, the LPF and the FSF decided to end the boycott.
[Dates? Other events? Why did Xerox's case against Apple fail?]