They don't even have to be hardcoded, they just have to ignore anything you specify or not give you any option to specify your own. As long as a device manufacturer can push updates to your device (even by IP address) they can regularly update their chosen DNS servers when needed. Honestly though, for many devices I doubt they'd even bother. Companies seem to have little trouble taking the position that if your device is more than a few years old you're insane for expecting them to still support it and you should have already thrown it away and bought another one.

League of Legends hardcodes 8.8.8.8.

Thank you for the example, probably 53/udp, which one can set up a NAT rule to direct all outgoing 53 to the local filtering DNS resolver.