Welfare story of the day - am I wrong?

I'm in a rush to get home after work this evening and stop at the grocery store to pick some things up for the wife.

I choose the one line with only one person checking out. It's a mom and her two chunky daughters in public school uniforms.

Thinking I'm in like Flynn for a speedy checkout, I realize the lady is having some sort of issue with the bag of chips she's trying to buy. "Same thang happened las week too. I don't know why it does it but the las girl fixed it", she tells the cashier. Irritated, I try not to make it obvious that I'm looking at other lines to see which one I could escape to. Then, just as I grab my things to go to another line, the mom tells me I may want to try another one.

I get to the line right next to my old one and as my guy is ringing me up, I realize that the lady's problem is that the chips aren't ringing up as a qualified welfare purchase. I instinctively turn around to grab it and buy it for her (thinking of the embarrassment this lady must be experiencing) when I suddenly realize her kids are fat. I'LL BE DAMNED if I'm gonna contribute to her daughters' obesity. She clearly stated she bought them those greasy chips last week too! I quietly turned back around, paid for my shite with my debik card, and walked out.

So my intended good deed turned into a somewhat political statement/bad human thing. Am I a bad person? What would the OTdo in this situation?