“No person shall display the flag of the United Nations or any other national or international flag equal, above, or in a position of superior prominence or honor to, or in place of, the flag of the United States at any place within the United States or any Territory or possession thereof:
That makes no mention of “government offices”
The law says "should," not "shall." It's a recommendation made to everyone, but only binding on government offices (assuming the government chooses to make it so).