Optimus needed someone who knew the law and who wasn't willing to skirt around it just to get the desired results.

Like Prowl, who served as prosecution.

And yes.

Prowl was not happy.

[He may or may not be cracking a slight smile at that.]

Megatron and Optimus had an arrangement. He would allow Megatron freedom of movement if Megatron publicly denounced Decepticonism to the entirety of Cybertron.