Apple did not "rip-off" the Macs UI from Xerox. Apple had hired some people from Xerox (like Jef Raskin, Bruce Horn) who believed in concepts of a Graphical User Interface. These concepts are pretty broad -- like making a computer easier to use by using graphics (icons), using menus, windows and making a consistent interface to do things. The work on these concepts predates Xerox PARC -- in fact it was many of these peoples individual work on those concepts that got them hired at PARC. So Xerox (PARC) brought them together to refine them.
Apple's work on GUI's predates Steve Jobs visit to Palo Alto Research Center. Apple had already had the same broad goals of offering an easier to use computer, and possibly using some of the same concept (like menus, icons, and graphics).
Jef Raskin had worked at Xerox, and he was tooting the "easier to use" trumpet, with his vision of what that meant. He brought some of those ideas from Xerox, but he had brought some of those ideas TO Xerox as well. Later, he convinced Jobs to visit Xerox PARC, and Jobs became an immediate convert (for ease of use).
What Jobs saw at Xerox was a prototype Smalltalk development system. He did not see either a working ALTO or Star (which was developed much later).
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Jobs was so hot on the concepts of UI, and the living Demos he say, that he, later, negotiated a deal with Xerox. He gave Xerox a large sum of stock in Apple (worth Millions) if he could come back, and bring some programmers -- to inspire them more on the concepts of GUI. This was like a one-day tour. This was agreed to by Xerox, and so by no stretch of the imagination could this be called "ripping-off"
