If the owner or authorized agent of a structure or whatnot as worded in the law, be it a business, gov't bldg or whatever, orders you to leave the property/tells you you are no longer welcome...you must comply or face charges. The time to fight it is after you have complied unless you want to wind up in jail. If you believe you were asked to leave for discriminatory reasons (i.e. race, gender, etc..), you are free to seek legal counsel regarding civil remedies. The part about signs probably refers to such signage as "Restricted Area: Keep Out, Employees Only, Authorized Personnel Only" or similar. If you are there after seing said sign, you are in violation.

