Looking at my iPhone 6 Plus, the curve seems to start at or just inside the edge of the screen. It's really close! My take is that with a firm glass screen protector, in the lab during testing, when they had it go all the way to the side it would peel up easily because it couldn't lay completely flush, and since it's so rigid they couldn't solve the problem.
I think these companies are going to have to mold the glass to fit the curves. It looks hideous even without the screen overlap issue. I think it's doable, but will probably increase the cost to manufacture which will be passed onto consumers.
Apple doesn't manufacture any screen protectors. It's all 3rd party. As a matter of fact, a few years ago Apple completely stopped selling screen protectors in their retail stores. As far as I can tell they reversed that policy at some point, not sure when. I'm not really much of a screen protector user since the iPhone 4. The anti-glare that I used to love looked horrible on the retina display. The iPhone 4 ended up getting a small scratch but it wasn't bad. Traded it into Amazon a few years back and got the best condition price, even though I selected the mid-tier due to the scratch.
Ever since the iPhone 4S the glass has been hard enough that it doesn't scratch, so I just go screen protector free. I'm still unwilling to go case-free, especially now with the slippery iPhone 6. Funny enough my whole family always has me apply their screen protectors because I'm really good at not getting any bubbles. It's all about turning off the A/C and waiting 30 minutes, then using a clean microfiber and some scotch tape to dab up any particles as I slowly lay it down. Bubble free every time!