Unions don't dictate your pay, they set a range and the company pays in that range for performance. It's no different than what your employer does to you today, except with a Union the ranges are known and negotiated collectively instead of the company dictating them.

Unions don't prevent people from being fired, they just require cause. Because the flip side of this is corporate firing people without cause days before the qualify for 401k matching, pension plan, or another stock vest or other such nonsense. You want to fire Bob because you don't like him? Tough luck, we're not here to be friends, we're here to work. Union says show evidence Bob isn't doing his job, then you can fire him.