Because they're all too chicken.

My sister even say's she doesn't approve (although she wouldn't admit that to my mother) but say's her kids are too old for her to be trying to tell them what to do and not to do. They probably wouldn't listen to her anyway and it would probably end up in a nasty fight.
IMO it isn't just leaving the kids home alone and them behaving or not behaving, but I worry about something happening. What if they decide they're hungry and try to cook something on the stove, matches? What if someone broke in or tried breaking into their house while they were by themselves? What if one or the other got hurt somehow? Do you really think a 10 year old is old enough to handle such of an emergency?
I don't think my mom was out of line for saying something. She didn't say it to be bossy, but because she was concerned.