I kind of get it. Stores don't want gun ho employees grabbing people and causing a lawsuit, don't want to escalate the situation to where someone might get hurt, or if some white employee stops a black guy and all the blacks come out of the woodwork yelling racism.
But firing? That's way way out of line for someone trying to do the right thing.
Lowes should chastise them, then go in the back room and thank them and give them a day paid time off. If I was a employer id be happy to have decent moral employees that care about right and wrong and are looking after their employers best interest.