Well, as long as that "lady" who is a Best Buy employee in a Samsung shirt and is making probably $12 an hour says so it must be true.
Samsung doesn't sell phones directly in the U.S.; only through the carriers which are responsible for their support and warranty repairs. They will do direct repairs for carrier phones so that group within Samsung's policy on the KNOX bit is the only "official" one. So the Best Buy lady isn't really a barometer for Samsung's policy. Maybe the U.S. carriers do have individual policies regarding the KNOX bit and a device's warranty, maybe they don't. You'd think with the N3 being out for seven months if any of the U.S. carriers were taking a hard line on KNOX we'd have heard by now. Same thing with Samsung's U.S. repair center which repaired and reset the KNOX flag for an XDA member as referenced in my earlier post
It's been asked before, does anyone have links to posts where Samsung's denied obvious defect repairs solely because the KNOX bit was tripped? I've seen fear of it happening, and reports of threats by Samsung or their reps that it could happen, but haven't seen any documented cases where it has. I have seen quite a few "Score, Samsung repaired my device even though KNOX was tripped" posts.