I think the parents did exactly the right thing. They know full well that if they had prohibited alcohol, the teenagers would simply have gone somewhere else, where they could drive afterwards, and which could therefore be very dangerous. The fact is, the teenagers were probably going to drink alcohol in any case: better to be under supervision.
What would you feel more secure about: that your child was drinking alcohol at a friend's house, under supervision, and that he would definitely not be driving afterwards, or hitch a lift with another drunk friend, or that he was out drinking in a seedy nightclub, with every possiblity of drink-driving himself or being in the car with someone else drink-driving - he could be killed.
The authorities need to understand: teenagers drink alcohol. Fact.
Any amount of laws is not going to stop that. However, what you can do is allow parents to try and make drinking safer for their children.