Actually, the legal aspect of character ownership on TV was very different in the 60's and 70's. Writers often got to keep overall ownership of one-time characters to earn residuals if the studio reused the characters in later productions.
Harlan Ellison owns the characters he created for that episode of star trek in their filmed appearances meaning any filmed appearance done for profit must kick him some money.
Theodore Sturgeon owns the rights to T'Pau character and was originally to be a main character on Enterprise, but due to legal concerns (royalties) the character was turned into T'Pol and the T'Pau character was recycled for a one-time appearance on Enterprise and background mentions in previous series.
http://memory-alpha.org/en/wiki/T%27Pau#Background