Is it legal for your employer to cut your wages because you voted for a union?

No.  This may be both a violation of the union agreement and an Unfair Labor Practice.  The ULP claim may be filed with the National Labor Relations Board if you are not a government employee.  If you are a public employee, it could also be a violation of the First Amendment to the United States Constitution.

No, it is not.  It sounds as if they retaliated against you for execerising the most basic of constitutional rights.  Are you sure and can you prove that this is the reason?  Then seek help from an attorney in your area.  Good luck to you.