Some argue that there is not much of a differece between Unix and Linux, since they are based on the same APIs. However AFAIK you need a certificate to be able to call yourself UNIX, and linux doesn't have that, since it is basically free. I think the problem is philosophical more than technical. Linux, or by his full name, GNU/Linux (where GNU means "GNU is Not Unix") believes that software should be free.
Read the GNU Manifesto for more information:
you can also check this page: