not defending the practice, but backbills are a common practice and what may or may not have been an oversight during transition is not an excuse for free passes.

people who are good with money would recognize they've been underpaying for the last six months and would either A.) bring it to someone's attention (save me your "YOU'RE LIKE THE KID SAYING TEACHER DIDN'T COLLECT HOMEWORK" fingerpointing crap, you know it's the right thing to do and will save you a headache in the long run) or B.) make a mental note and don't go spending above your means for when the backbill inevitably comes.