There's no moral high horse here, no 'rights' being outraged.
A union is an association with an agreed set of standards for its members.
Racism in a general association is not only 'unacceptable' it is physically incompatible with the aims of any body working in the collective interest of its members.
And more than that, collective bargaining is politically incompatible with the rest of the BNP's right-wing, supremacist agenda. So everyone should be ecstatic that their paths will never cross.
If racists want to set up their own racist union, good luck to them. They will then have really eaten their own tails.