Presumably they restrict the distribution to make sure only their dealers can do installations.
I maintain a library of avionics documentation for equipment our flying club either has installed or might install, and the install manuals are useful reading for background, and in planning future upgrades. I'm guessing somebody will leak it online eventually (that's how I got the ones I have now); I don't have specific questions and understand your desire not to piss off Garmin or get your dealer in trouble.
