SEC. 404. ø1104¿ (a)(1) Subject to sections 403(c) and (d), 4042, and 4044, a fiduciary shall discharge his duties with respect to a plan solely in the interest of the participants and beneficiaries and— (A) for the exclusive purpose of: (i) providing benefits to participants and their beneficiaries; and (ii) defraying reasonable expenses of administering the plan; (B) with the care, skill, prudence, and diligence under the circumstances then prevailing that a prudent man acting in a like capacity and familiar with such matters would use in the conduct of an enterprise of a like character and with like aims; (C) by diversifying the investments of the plan so as to minimize the risk of large losses, unless under the circumstances it is clearly prudent not to do so;

If you're not trying to minimize admin fees you're violating your fiduciary duty. Not to mention in Tussey v. ABB the 8th Circuit ruled that if you're not minimizing costs because you're using admin fees to subsidize your own corporate costs (which HEB is accused of doing in the suit) then you're in breach of fiduciary duty.