Because it's a democracy and let the majority decide
ROTFLMAO
In which case, why did Len McCluskey et al not consider this to be the fundamental principle of the union (which in theory it is) and postpone (
note: postpone not give up) announcing strike action until he had given the membership the opportunity to make a democratic decision? A "no" vote then would have strengthened his hand.
Moreover, why did he prejudge any outcome by saying that the union were not minded to approve the offer?