英語の勉強(What I don't understand is...)

What I don't understand is why some customers prefer disposable plastic containers when they're staying in the cafe. That generates more CO2 and wastes resources - don't they know? The cafe staff recommend using ordinary reusable mugs, and we should follow their advice. Many of those who insist on plastic cups seem to be germaphobes or something like that. If they absolutely need to use them, that's their choice, and I shouldn't say anything. But otherwise, I wish they'd switch to reusable ones.