A certain type of person (one who reads and understands the rules, as well as has lots of experience) gets modded.
Similar people tend to group together and be friends (and pairs)
Thus, mods are friends (and pairs) with people that read and understand the rules, as well as have lots of experience.
People that read and understand the rules, as well as have lots of experience, tend to not get infracted because they know the rules.
Thus, mods' friends and pairs tend to not get infracted.
I'm pretty sure that's closer to the logical reason for the lack of infraction (how would you know, anyway ._.) of mod's friends.