what would witnesses do ..........
if they were treated this way for being JW's. - They would sue petsmart for as much cash/publicity as they could muster. Their lawyers are always busy dealing with 'discrimination' against them.
But you can't actually sue the Org. for this one. Although they encourage the mentality that was displayed here, they would point out that no Watchtower article specifically said dogs of DA'd people should be shunned. Behind the scenes, they would praise the actions.
Report the employee, and also say you are taking legal advise regarding suing the company for damages. Soon you will need someone else to look after your dog because they will have been dismissed!
Then they will relate their story of 'persecution' at the local Convention - how they lost their job for upholding God's standards. No mention will be made of the 'persecution' they inflicted on you and your dog, and no mention of scriptures to back up their behaviour, since they don't exist!