This safety gave the Bills an eight-point lead with under two minutes left and the Packers would never see the ball again. But it doesn't look like a safety:
It seems silly, because Lacy clearly makes it out of the end zone. But this was the correct call by the referees. There's a rule on the books that on a fumble with under two minutes in the game, the only player on the offense who can advance the ball after a recovery is the person who fumbled.
This rule is on the books because of the Holy Roller play, on which the Raiders blatantly and intentionally fumbled the ball forwards with the clock showing zeroes to get a game-winning touchdown. The rule prevents this obvious bit of chicanery from ever happening again, which is good, since that isn't exactly in the spirit of the game.
This rule is only kind of a bummer most of the time. If this happened in the regular field of play, it only would've inconvenienced the Packers by about 2 yards. But since it happened in the end zone, it was a safety, and they had to kick the ball away instead of facing second-and-long. They tried an onside punt on the ensuing free kick, but it was no use.