iOS is like that too. It keeps all programs running by default, and then when memory gets low it'll start killing them. It starts with backgrounded apps that aren't doing anything, and kills the current foreground app as a last resort.

Apps are supposed to be built to tolerate being killed while in the background and restoring their state when relaunched, so in theory this will mostly be invisible to the user other than apps taking longer to reload. In practice, lots of apps don't get this right.