Who owned the property, if it wasn't the owner?
It doesn't matter who owned the parking lot property. The woman was an employee and there were no restrictions on employees parking there cars there. The car belong to the woman and the bumper sticker is a semi permanent attachment and a display of her federally protected free speech right under the 1st Amend. The employer intimidated her, threatening job loss if she didn't remove the sticker. That's a fed crime under 18USC245 as posted above.