According to the game, apparently, it is okay for people to use Machina for recreational purposes (Blitzball).

I would imagine that Sin does not cause total destruction, as it would end the point of his existence. If Yu Yevon wants control, he's going to get it, but you can't spread your destruction completely without invalidating your own existence. Ohh yeah, the whole premise sounds Machiavellian, but it works.

Additionally, the Crusaders work the hardest to make sure Sin does not attack Luca, while Bevelle is protected for being the center of Yu Yevon's wholesome religion. For the sake of argument, I'll ignore FFX-2's "contributions" to the sociology of Spira, which more or less paints the inhabitants thereof as complete morons who wouldn't have needed Sin to kill each other off. By focusing their fire on Sin, they probably kept each other alive.

@ Renmiri, people can survive repeated catastrophes all the time. Ever heard of TEH J00Z?