Mods know the rules
The power of a mod is often confused with the responsibility of being a mod*. Mods don't close/open a question because they feel like it, they do so, because they know what the SE standards are.
Normally, you will only see a diamond close on a question that is clearly off-topic, otherwise you will almost always see a comment explaining why they chose to close it.
The fact is that it's a mod's job to make sure the standard is kept. The community is usually in charge of weeding out the rubbish, and mods step in when things get a bit much. But the fact is they are part of the community already, so it's not like they're above and out-of-scope with everybody else.
While yes, I can understand that sometimes this sort of "power" might seem a bit unfair, but it's really not. It's a responsibility that is given to them, by the community, for the community**.
All in all, the system, as it stands, works 90% of the time, and if it doesn't it is always dealt with appropriately.
A good example of this is actually one of my latest questions***. While some would consider it a good question - on topic, answerable; it could quickly get out of hand for some who might see it in a different light. A mod stepping in and closing it immediately removes that confusion, allowing for it to be dealt with appropriately.
*Uncle Ben is always wise
**I also apologise for all the Patriotic talk
***Only viewable to people with the appropriate permissions - has since been deleted