Geez, not this shit again. (One of the many ways you could tell Oracle was doomed in trying to find and convincingly argue a tenable position against Google: half the people half the time argued that Google should be sued for implementing Java, and the other half the time argued that they should be sued for not implementing Java.)
Yes the external format of a jar file is a zip - but there is also a specification for the internal structure and file formats, as well as information like the minimum java version required to execute (and all sorts of other crap, I can't really say since I haven't actually looked - these are just some sane enough minimums).
