Because the girls "don't want to be shamed" at school.
Because enviromentalism is the religion taught in public schools, and it's the kind of religion done with shaming young people.
1. Some people wash and reuse ziploc bags. So don't presume you know that the ziploc-user is an enviro-sinner.
2. Kiddies, if you are old enough to understand environmentalism and to pressure your mother with it, you are old enough to pack your own lunch. And if you're so hot on being saintly, start helping your mother, not making her life any harder.