Because it's an effective tactic against exploits that can't survive a reboot, which is somewhat common from my understanding. The idea being that police can confiscate your phone and just keep it on and charged until they can buy or develop an exploit targeting your current device and software.

I was admittedly confused about this distinction at one point too. It's a trade-off (although few people effected by this own phones with truly free, user-respecting soft/hardware in the first place).