The fee *could* stand legally, in the sense that there is no law against charging an administrative fee, or a greater one to non-resident co-owners. However, it may not be imposed at will. It would need to be something that's in the HOA agreement, the bylaws, some other contract or agreement by which you're bound, community rules, etc.--i.e. there needs to be some contractual basis for it. It may be the case that the governing documents allow the association to change or add fees at will,  in which case they can do this. If that's not the case, they could only add it whenever the governing agreements or rules come up for renewal. If you haven't already, ask the association to show you or point out the basis for the fee; take a look and see if makes sense.