Apple is a closed operating system while Android is open. Carriers are able to customise and optimise phone software that they provide for their network because of android's open nature, which is not possible on Apple devices.
It's not picking and choosing. Samsung only allow access to the phone's bought directly from them because they DO NOT need any carrier input. It's the software they have complete control over. If it were open to carrier modified software then Samsung would have to wait for said carriers to also test their modifications on top of what Samsung themselves are already testing.
Can you imagine the massive delays having to wait for each carrier to approve their software?
