That's an aggressively stupid post, Shinichi made a clear and coherent point which Migi accepted. It's in both of their best interests to cooperate with each other as much as possible.
He could have made the same argument for telling his parents or other people. I even went to lenghts to explain that several weeks ago. You told me why Migi wouldn´t agree to that. Now about a single sentence excused Shinichi´s disobedience. I mean, come on.
"She's the only non-loli girl in the show, your honor!" will be my defense in court