Sorry to double post, but like I said, given how beat up the front is, IMO, the most likely explanation for the back sticker missing is that the previous owner just didn't take very good care of the game.
considering it's from amazon, If I had to guess they replaced the back for a nice one.
Most folks rather have a nice grey back than a discolored one with a shitty condition sticker. most folks would less likely return said cart than.
I'd imagine the original back was in rough shape considering what the front looked like.
Still I wouldn't really think to deeply in why the back label is removed. as long as the front lable and the board is legit nothing else really matters.
I read a site on how to tell if a SNES cartridge is authentic, and my copy of Link to the Past passes every one other than the back label and the board (I refuse to open up the cartridge). It has the product number code where the pin connector is, the game title on the label is clear, and the dot for the "i" in the Nintendo logo on the back is a square instead of a circle. And let's be real, it'd be hard to fake that kind of wear.