"UNIX" is a registered trademark of The Open Group, so systems such as GNU/Linux, Mac OS X, FreeBSD, NetBSD, and OpenBSD can't use it without paying royalties that would be too expensive for a product marketed to individual consumers. Nevertheless, the term has been applied to these systems in much the same was as "Band-Aid" is applied to all bandages or "Xerox" is applied to all photocopiers. The spelling "Un*x" was then derived as a way to say "Unix" without really saying it.
When ordinary consumers refer to their generic-brand facial tissues as "Kleenex" or their Linux boxes as "Unix", this falls under fair use. Nevertheless, many individuals stick to "Un*x" in writing for various reasons, such as: