Is a vote a bad thing?
In general, no. In this specific context, they may be trying to grind the union down by forcing them to vote on one unsuitable offer after another until they ratify one because they're tired of voting or because of government pressure, instead of attempting real negotiation. I don't think it's going to work, but they may be trying.
Declaring work-to-rule rather than a full strike was a very smart thing for the union to do here—it reduces the other side's options for applying pressure. Can't have the government order them back to work when they're already working. Which, I'm sure, is why they did it.