I wouldn't say that's always been the case. I used Mac OS from 7.6 to OS X 10.6. It never stopped you from installing programs from whatever source you wanted, even if the hardware was controlled and most applications did all look pretty similar due to the provided UI libraries and the strict UI guidelines. You were also able to mess with and delete random parts of the OS; I managed to break Finder entirely, which is how I ended up learning how to use the terminal and vim.

It was Mac OS X 10.7 when they disabled unsigned apps by default to try to force people to use the store and pressure developers into paying them an annual fee. That was also when I stopped using Mac OS.

I was really happy with Apple ruling their little UI fiefdom with an iron fist because there were alternatives. I do not like them having total platform control down to the silicon.