http://www.slate.com/blogs/moneybox/2012/06/07/sometimes_a_pension_cut_is_just_a_pension_cut.html
Municipal governments, in particular, have no real ability to impact national and global macroeconomic trends. The relevant decisions are above their pay grade. If macro stabilization policy fails, all local authorities can do is decide how many unexpected tax hikes and unexpected spending cuts to make.
Consider it from the other direction. Unions would, for obvious reasons, rather not have their pensions cut. They might prefer tax hikes as an alternative. You could go around wondering why labor unions have all this hostility to taxpayers, why are they always trying to get their money, etc., but the truth is that there's nothing to be explained here.
The union isn't hostile to taxpayers, the union is hostile to having its pensions cut! Conversely, if voters choose pension cuts over taxes that's not because they don't like unions it's because they don't like taxes.
The entire debate is about raising taxes on 1-3 percent of the population. Municipalities operate with a much broader and more regressive tax base than the federal government so that option's not open to them. Consequently, a bad economy leads to reduced spending on all kinds of things. Including pensions. Which is bad for the workers whose pensions get cut and naturally unions will fight back.
And to a deeply political person that may look like a controversy about the decades-long decline of American labor unions and the macro structure of American politics, but to a normal person it's just a question about paying more taxes or less.