Expired lettuces

So, I was at the grocery store tonight, trying to make a record for how quickly I could get in and out (have I ever mentioned that I DREAD the chore?), when I encountered something that gave me pause. A produce stock person was somberly throwing bags of lettuce and other greens into a cart. It was piled high and from what I could tell, it looked fresh enough. The guy grumpily said as if to himself, "This is what I hate about my job. Thinking about all the people in the world that don't have enough food and how much I throw away each week." I said something sympathetic and continued on with my list. But I continued to think about the incident as I finished shopping. Why don't grocery stores sell the food the day it's expiring rather than throw it out? As a person on a budget, I would appreciate that practice. More importantly, why is there shameful excess in America while so many across the globe live in poverty. I think, what can I DO?