It is actually quite simple. Whenver you see a trinitron tube in a non-Sony monitor, that tube was usually OEM'ed from Sony. Viewsonic, Mag Innovision, and so on use Sony tubes in their monitors, with their own support electronics. Sony still manufactures the tube, so they get a cut. This benefits both Sony and the other manufacturer; Sony sells more tubes, ahd the other company sells more monitors because people tend to like Trinitron technology.

Gotta love big business. Things are different in the TV arena, though. Since volumes are higher, and people aren't so picky about TV picture tubes, companies can make more money by simply using a tube from another manufacturer.