In an attempt to squeeze more cash out of the carcass of Sun Microsystems, Oracle made the the terms of the Solaris 10 licensing agreement more restrictive. In 2005 Solaris licensing was made available for free. Sun made money by charging for support and maintenance. This was done to make Solaris more competitive with Linux which was eating away at Sun and other UNIX OS market share.
Today $olaris 10 is available for a 90 day trial. After that you buy a license.