In the case of Debian I think it is philosophical. It's been years since I've had to install proprietary things on Debian, but they used to be all in the non-free repository that you had to add manually. Honestly I like it, it reminds me I'm putting proprietary crap in the machine. Can be a pain in the ass when the wifi doesn't work because some proprietary firmware is missing, and the laptop doesn't have an Ethernet port so off you go to buy a usb-eth adaptor.