Why not? I mean even if Japan is doing very good, letting people go in and out would mean that it can turn around very quickly!
I for one applaud Japan for keeping restrictions up. We're in a pandemic that isn't going to go away soon, especially not with people touring the planet whenever the gates open for a second, spreading or importing the virus from god knows where.
Edit: Like tourism is a huge factor in covid spread. You can see it happen live as of right now even. Cases start to go back, measures start to be dropped like travel restrictions, people swarm the world like rabbits and come back and, who would have thought, cases skyrocket once more and everything has to shut down for the 4th time now. Imagine where we would be now if people wouldn't mindlessly start to tour the world trying to catch all the covid strains.