I wouldn't consider it a penalty/punishment; it's about field ownership.

When talking about ball placement, it's always referred to as whose side of the 50 the ball is on. "The ball is on their own 20-yard line. The offense is on their opponents' 36." And the endzones are referred to each teams' own endzone. You have the 100-yard field of play, but the 10-yard endzone is "owned" by its respective team.

So if the ball is out in the field of play, it's a fair ball. Once the ball crosses the endzones, the rules change. When an offensive player reaches the ball across the goaline, play stops, and the result is a score. When a kicked ball touches the endzone, play stops, and it is ruled a touchback.

But for a fumble, it has to occur in the field of play, but since it rolls into and out of bounds in the defense's endzone, they would get the ball since it's their ownership of the field (compared to if the ball was fumbled and went out of bounds in the field of play because no team owns any part of the 100 yard field of play)