I'm thinking this defect wasn't on all 3DS'. Mine is a launch 3DS, and i've had no problems with it. Ditto for my friend. I open and close my 3DS a LOT, since I play it a lot when helping my dad with his job (which, in a nutshell, is him driving us from location to location, with me constantly getting out, so I open and close my 3DS at least 75 times a night).
But anyway, you can do a whole hell of a lot if you're good at social engineering. The people who got it replaced for free were either lucky and got a nice employee, or knew how to talk to get their way. People who had to pay either got an incredibly stubborn employee, or were submissive. If they are adamant on having you pay, hang up and try again later.