My guess would be that the three-day timer is the first version to test the waters. Put it out there to see if there are any unexpected problems. And if everything's peachy, lower it in some future release to make it more secure.
Apple’s scale is so large, it’s easy to forget even tiny percentages are still actually really significant in absolute terms.
I can see this being too hard to explain compared to “reboots after 72 hours.”