I can only imagine that you and your DH are not the only people to have noticed - surely over the years other people have said something to them It's truly negligent parenting.
I'm guessing that they think they know best and don't care for the so-called 'rules'.
If you don't think you can have an upfront conversation about it, I'd consider reporting it anonymously. It's better than doing nothing at all.
I'd personally be tempted to try and give them a seat, even if that meant buying one second hand of buy/swap/sell. I guess even then, it's no guarantee they'll use it just because they have one