The X Window developers were mostly just making it up as they went along. Practically no one expects mouse buttons to work like that.

Unix workstations had mice with 3 buttons. The Mac had only one. Windows, Amiga, Atari had two. The Unix developers had choice that others didn't have. They came up with a use that existed forever. Now someone decided to remove the default for no apparent reason. It's like the Android product managers continuing to change the color, size and gesture to answer a phone call: every release, the first call is an exercise in managing frustration.