Safety Concerns
The point you are missing is that they just have, and that currently, under our legal system, they are allowed to do so, because you are allowed to do anything that is not explicitly unlawful. Unless that's taken to court and the union win - which is anything but certain given the highly debatable nature of the law in this area - then you can't simply wander on here and say that they can't do it. Just because you want something to be so doesn't mean that it is.
Right now, the airline can. If the union want to take the massive risk of going to court, then it's possible in the future that BA can't.