First, they're unionized. Understandably, the union is against unpaid labor of any kind. It sets a precedent that is more problematic than anything.
Second, we have a Federal minimum wage. You're not allowed to make less than that in a job. Therefore they would have to be paid something to still be in compliance with the law.
Finally, while one person can go into an office without incurring massive costs, if you have an entire office come in, then you need to make sure receptionists, janitors, kitchen staff, parking attendants, etc are on the job.