Why shouldn't they relax if they can? It's bizarre to me that we need to regiment the entire population in order to... make sure people don't have fun?

Whether it is human nature, or learned behavior, the typical human is unable to fill their time with productive use over a longer period of time unless purpose is imposed on them. Hence "work". The concept in a more general term is even mentioned in religious text so there must be a basis....

You see what you seek. I saw amazing creative output by many many people. It is telling that you reference religious texts as evidence for something.

This is puritanism. There is something to the idea that idle hands make for the devils workshop, but that's mainly when people are dissatisfied. If they're satisfied, let them play pokemon cards or whatever it is they do. You only get one life.

I believe at a higher level, a universe level, rather than a human level - human natural state must be an equilibrium of "energy" & detriment. (energy is a meta concept crossing domains here, stay with me).

Humans consume energy (animals, resources) and create detriment (greenhouse gases, etc) - and must offset all of those to positively benefit the Earth or else that (and each) particular human is a net negative to the Earth. If a human is a net negative, at a higher level why should that person exist?

Humans must each therefore cause a positive net of energy to the Earth through creating useful, productive, additions to the universe to push it forward. Otherwise, as in the matrix, said human is a parasite.