Its an interesting question. I'm not a union guy and spent my youth working in construction so I've encountered my share of plodding union tradesmen who took remarkably long to do small things that could be finished in a few minutes if you rushed. When I was young and foolish I'd often shake my head at the snails-pace they worked at.

I'm older now, and arguably a smidgen wiser. I've reacted with shock at hearing about old acquaintances who have been killed on the job or now endure chronic pain and are no longer able to work. Broken backs, bum knees, busted nerves, missing fingers, the list goes on. The plodding, methodical 55 year-old union guy is looking less and less silly the older I get.

In short, perhaps one reason a union might push back against performance-based bonuses is that it often ends up coming at a long-term cost to the worker that is difficult to foresee when you're an invincible 25 year-old trying to make a few extra bucks for the weekend.