“This case concerns the question whether, by promulgating the fiduciary duty rule, the Secretary overstepped the bounds of the authority granted to him under MUSA. We conclude that he did not,” Justice Dalila Argaez Wendlandt wrote on behalf of the court. “We further conclude that the fiduciary duty rule does not override the common-law protections available to investors, that MUSA is not an impermissible delegation of legislative power, and that the rule is not preempted by the Securities and Exchange Commission’s (SEC) determination to impose a national ‘best interest’ standard of care on broker-dealers, 17 C.F.R. § 240.151-1 (2019) (Regulation Best Interest).”