It's pretty clearly not intended to cause any harm, direct or indirect. It's use, broken down to it's most basic, is trading the caster's turn for the enemy's turn. Weather or not to allow OA's on Approach/Flee I think is a DM decision to make for the feel of his games. It does make the mostly harmless spell more powerful, but not as powerful as say, Burning Hands, as a damage spell.
I disagree. The text specifically says, "directly harmful to it." Indirect harm does not come into play. Commanding an opponent to Grovel makes them go prone. In the middle of a group of your allies that is a Bad Thing (tm), but the harm is potential harm from giving all the adjacent enemies advantage. Grovel, Approach and Flee on their own are not directly harmful, but a target on a single rock in the middle of lava will not approach or flee if all it can do is walk because
it would directly hurt itself to do so. Prone would not directly hurt it in that situation, but it could slow it down to the point where it does not escape the advancing lava.
With Approach and Flee, the commands are not directly harmful to it if they do not cause the target to hurt itself. Approach across a bottomless chasm would not work, nor would Flee up against a cliff because of a fall. But Approach and Flee to leave an enemy's reach are not directly harmful. The target still gets to defend itself as evidenced by its AC not being penalized and advantage not being granted. There is a potential of harm, but there was also a potential for harm if the enemy had stayed still.
The target gets three chances to avoid potential damage from an opportunity attack.
1. The target makes the Wisdom saving throw.
2. The adjacent enemy decides to not use its reaction to make an opportunity attack or already used its reaction.
3. The adjacent enemy misses on its melee attack.
There is so much going on here that it can easily be argued that Approach or Flee cannot be ignored if the only danger is an opportunity attack.