Well, presuming we're talking about alcohol some parents will 'allow' their kids to drink booze and others won't. The host parents seem to be saying kids can bring alcohol but they're not allowed to share it with other kids whose parents don't want them to drink.

So how is this going to work practically? Are the host parents going to monitor who came in with alcohol and who didn't? What if they see a kid sharing whose not meant to have alcohol? Who is held responsible if a kid gets drunk who promised their parents they wouldn't drink? Is it the hosts' 'fault', is it the kid who shared their alochol's fault or is it the kid who took the alcohol for breaking their promise?

I just don't think it's very practical.