A private company has a right to set a policy prohibiting such expression, or not to have such a policy. Where it would get ugly would be either requiring the expression of the owner’s beliefs or enforcing the policy differentially depending on what party it was. But even that is apparently legal in all but a few places: “Unfortunately, it is legal for an employer to make employment decisions on the basis of an employee’s political participation (or lack of participation), unless the employee is in a state where making employment decisions on this basis is against the law.” From here: http://www.workplacefairness.org/index.php?page=retaliationpolitical#5
If you want to complain, start by asking if Penney’s has a policy about it. As a retail (largely) clothing store, I would think they probably would, since such stores tend toward really picky dress codes. If they don’t, and you support limitations on freedom of speech, or just want to take every opportunity to make life difficult for this lady who (I assume) supports the opposite candidate from you, complain that they should have such a policy.
Stunningly inappropriate, to me, would be the US government telling private business owners they have to prohibit political speech by their employees.