It's a little more complicated. The Supreme Court has already ruled that Trump can fire agency heads not for cause even when there is a "for cause" clause in the law. The Court then invented a new "the Federal Reserve is special" clause that prevents Trump from using that exact same power on Fed Governors. So because the Court has made a patchwork quilt out of the law (in an effort to bring about their preferred policy outcomes), Trump is now trying to muster "cause" against Cook. Early signs indicate that the case appears to have serious problems.