Strangely, I feel like even businesses should be behind this idea. Foodservice companies like Sodexo, Aramark, Sysco, etc should be pushing for this 100%. School food (like any food) doesn't come out of thin air. "Lunch ladies" aren't going to the grocery stores to find what to make. All these school districts have contracts with a foodservice company like the ones I mentioned to provide the food and meals to school cafeterias.
By providing free meals to students, governments are also providing guaranteed revenue to these companies. Because now the school is providing ALL the lunches for kids, whether or not some kid still come with a lunchbox from home. That's gotta be more revenue than before. Admittedly, I don't know that for sure.
To me, it seems like a win-win. Kids aren't going hungry all day, and big business gets to make its money.