All public sector unions negotiate their pay packages with no taxpayer representation at the table. In these negotiations your elected rep is now a gubmint representative, not your representative. Since the gubmint considers tax revenue to be infinite, there is no pressure to limit the union demands. This is the primary reason (among many others) to abolish public sector unions.
Government unions are negotiating with a company with unlimited resources (as you said, their taxing power). They never take pay cuts, and get annual Cost of Living increases virtually guaranteed every year.
P.S. Not to mention job security and pensions the private sector can only get in their wildest dreams..