It is a fine ruling. Teleporting while standing is safe {other than the mishap %], Teleporting while falling is not.
Does an object with momentum maintain that momentum if teleported?
Nothing in the rules suggests that the rules of physics wouldn’t continue to apply, so it appears that momentum is maintained. If you’re plummeting toward the ground when you cast teleport to reach a safe spot, you’d still be “falling” and would therefore take damage as appropriate to the distance you actually fell before teleporting.