Because I agree with you in some ways. Dogs use their mouths to do everything, including communicate, which in comparison is our hands. They use their mouth for everything. From the sounds of it you have some really trained dogs and know what your doing.
On the whole 'they know what they're doing but do it anyway', I stand in the middle. If your dogs are taught not to bite you, then they won't but otherwise they will because they weren't taught not to use their mouths on humans. Its that simple. I used to have a 120 lbs full blooded gs (a retired police dog) who would grab your hand and gently shake your hand without using teeth. He'd just want to hold your hand. He never once used his teeth and never bit me, so I understand that they know what they're doing but in some cases they just don't. One of my friends dogs that they received from a friend (apparently the owners maced him and let him go. He seems to have some kind of neurological problem and barks all the time. I'm guessing that's why they did that) does not have any manners at all. He's a young dog and in his excitement to greet you will jump all over you, trying to bite your hands at the same time. He doesn't get that that's not acceptable but can be made to learn.
For a dog its natural for them to bite/use their mouth and unless you break them of that habit, or they aren't biters in the first place, they're going to do it. Reminds me of my bestfriends cat who will bite the hell out of you but its affection biting. He doesn't do it meanly, he's just a biter and they've tried everything to break that habit, yet he still bites.
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