First, firefighters and police were not excluded from the bill; they were explicitly included under the rubric of "public safety employees" (as opposed to general municipal employees).

Second, the difference in treatment contained in the bill is that public safety employees can collectively bargain on hours, working conditions, and wages, whereas general municipal employees can only collectively bargain on wages. I'm no expert, but perhaps a firefighter sleeping in the firehouse so he can hop on the truck at 2AM faces different concerns than the secretary to the tax collector.