We had an informal work christmas party on the weekend. It was a BBQ potluck- we all brought our own meat to grill and a side to share.
We all bought our own booze, some of us who had wine shared with others, others drank their own because they didn't like what we had brought. No harm, no foul. BYOB is generally you bring what YOU want to drink and share if you feel like it. If someone asks if you want some of theirs, it would probably be considered rude to accept and not reciprocate.
Plastic wine glasses were put out but a lot of people brought their own glass. This is not too uncommon. It's normally laughed away with "at least I don't have to worry about dropping your good glasses!" A lot of us HATE drinking out of plastic glasses. It's not because we think we're better or looking down on the host, it's just a preference. I think perhaps Australians are less sensitive (not in a bad or good way, just different) to how we appear to others. We don't tend to get complexes about things too easily
Booze is quite common at children's birthdays too. Parents will often share a glass or two of champage. At such a young age, they really are more for the parents than the children.
The fact that they drank a bottle together and then drove home could be a worry, it depends on over how long a period the alcohol was consumed.