Everyone who is a moderator on the Stack Exchange network is completely voluntary and there is no pay. You may see employees with moderation privileges, but being an employee is not the same as being a community moderator.
Do moderators earn a salary?
For question two, Stack Exchange is also built on a system of community moderation using a reputation system the more reps you earn the more privileges you can gain. This means that the users who are doing those task you mentioned don't have to be moderators. The regular users can also do moderation task as well. Therefore taking some of the load off the moderation team.
Although it says Stack Overflow it still does apply to the SE network as a whole.
Stack Overflow is run by you! If you want to help us run Stack Overflow, you’ll need reputation first. Reputation is a (very) rough measurement of how much the Stack Overflow community trusts you. Reputation is never given, it is earned by convincing other Stack Overflow users that you know what you’re talking about.
This is taken from the blog post titled A Theory of Moderation.
A second case for question two is that a site may not have as much activity which can allow the moderators to watch over the site and keep things under control more easier.