So, are you saying, no matter who won the general election, before each piece of legislation is passed into law, we should have a referendum on it?
What a strange analogy. I'm not sure when unions gained the same kind of power that a government did...
But here's how things tend to pan out in a long running dispute. At first most of the membership is behind the union, the membership wants better pay, T&Cs etc. And they are prepared for a long one. But as time goes on, members start to feel the pinch. The cost of strike after strike starts to bite, it starts to cause issues with personal finances and even relationships. And in the workplace tensions rise as some member's resolve starts to wobble, pressure starts to grow between them and the member's still determined to see the dispute through.
When this happens it can start to become difficult for reps on the ground to simply gauge the mood by talking to members, especially if there is peer pressure to keep supporting the action. So allowing members to have their say at pivotal moments, say when a new offer comes across the table, gives all members the chance to have their say and help drive the outcome. If this doesn't happen, those members struggling to keep with the action will start to drift back. And when this happens, well it should go without saying but this gives the employer the upper hand not just in the current dispute but in future ones. I know because I was a TU rep in a long running dispute that went on too long and totally undermined the entire union because whilst the top table was still solidly behind striking, the membership wasn't.
It may be that ASLEF members are solidly behind it, and if so there is nothing to be lost by taking it to a vote. But if they are not and the top table keeps asking members to lose more and more income it could damage the union and it's members in the long run. Or they could be like the government and do what they want and stuff the consequences.