There's a difference between changing "if you work for us for another 20 years, here's what your pension would be" and "since you worked for us for another 20 years, here's what your pension is". As I understand it from the article, California's constitution prohibits changing how existing public-sectors accrue pension benefits going forward.

This is much more acceptable, IMO. It's the difference between "we're paying you less than you thought we would for work you did" and "we're going to pay you less for work you'll do in the future". There's an unethical opening for convincing people to take a lower compensation now in exchange for the right to earn a higher compensation later and then reneging on it (see also: firing people immediately before a vesting cliff).