OP is asking for a reason to NOT allow it to go up past horizontal, not why it should be allowed.
Sure, you could use Fn or C1/C2 (for remotes that have them) to center the gimbal, but first it will point down, then center. If you're recording, those flips might spoil your shot.
If you're varying your gimbal tilts during recording, you might want it to stop right at horizontal and not overshoot it. Manually changing tilt might otherwise be difficult to get it right on horizontal while recording.
It's rare I would want to go above horizontal so I leave it locked.