Directly from the Pentagon Spokesman Jonathan Hoffman:
Esper has also ended the Navy's plans to hold a review board, which would recommend to the head of Naval Special Warfare Command whether to revoke Gallagher's SEAL trident, Hoffman said."Secretary Esper's position with regard to UCMJ [Uniform Code of Military Justice], disciplinary, and fitness for duty actions has always been that the process should be allowed to play itself out objectively and deliberately, in fairness to all parties," Hoffman said in the statement. "However, at this point, given the events of the last few days, Secretary Esper has directed that Gallagher retain his Trident pin."
At the point it was revealed the board was compromised, he had little choice. That, and he's doing it so POTUS doesn't have to, like a good SECDEF should (and SECNAV should have).
More likely Trump jerked Esper’s chain real hard or he would have stood by and let Green retaliate against Trump’s pardon of Gallagher. I wouldn’t trust Esper as far as I could throw him.