I think most people understand mistakes happen.
Even on those "mistake = kick" groups, there is such things as understandable mistakes that players themselves can see them doing. One of those "haha yeah we've all been there" types of deals.
Those "mistakes = kick" descriptions are basically there to scare off the uninitiated, to help gather just people who know their rotation properly, and know the mechanics of the fight. There are times when you cannot, or don't want to use the [Duty Complete] filter, but you're ready for the clear yourself. A mention of a "Clear party" is basically just equivalent to a late phase learning party. Sometimes you need to put the foot down and put on your big boy raiding pants and get it over with. You may not want to deal with someone who is still much earlier in learning than you.

Of course Raid Finder has all these matchmaking options but NA groups never use that do we?