So far I haven't come across a good explanation for this restriction.
Logic is "If you can't move, you can't spread virus". In principle it is no different from restricting int'l travel, just on smaller scale. This is how people above think & what must be written in some 'pandemic cookbook', if there is one.
I don't agree with it either & think much better way to use resources is to clamp down on gatherings, masks, etc. The other day at Cascade Ponds picnic area in Banff: 10 or so picnic tables, every one of them 12-15 kids, blasting music, drinking, smoking weed, no mask any one of them. And they wonder why Banff is #1
covid hotspot in AB.