Excellent point made on this topic. Here is some more background info.
The Turner Library *was* originally made up of all RKO films, all pre-1986 MGM films and the pre-1948 Warners films.
The Turner Library, however, is no longer owned by TCM. Ted Turner, when he made the deal with Time/Warner/AOL a few years back, sold the library to Time/Warner as part of that deal. The Turner Library is now controlled by Warner Brothers. *This means TCM has to rent every film it shows*