Como
Here's to Loonyology!
In some cases people might owe someone a 'thanks' due to making them a sig, giving useful info or has helped a member in some way with the site. So that might explain why you see random posts getting 'thanked'
As for people doing +rep for +rep, it does happen, and there's not much that can be done unless it's brought to our attention, or we find it suspicious and we look into it.
Hope that's cleared it up for you....