That is routine in an accident investigation. I greatly doubt though that alcohol was involved in this crash. From everything I have read and heard and learned about that day and night, they weren't drinking. And you won't hear anything about toxicology on the passengers, it's the driver they are looking at.
I do hear your concern though about the underage drinking. These girls are/were normal, lives much like every other teenager, even when I was one. I've been to the memorial pages on facebook and looked at all the pictures of the girls and their friends. There are a handful, out of hundreds all together, where you can see beer bottles in the background, one where there is two cases of beer on a counter right behind the girls that are posing. Is that acceptable - NO! And, as a parent I wonder how did they get the alcohol, how did none of the parents find out (it was probably obtained by an older sibling or college friend). How did everyone get home from that party? Etc. But, I did those same things 25 years ago, I too can only count a handful of times in high school alcohol was at parties, my girlfriends and I took one sip and thought we were drunk. Even today, parents and kid's don't look at beer and wine the same way they do at marijuana and meth, cocaine and heroin. We talk so much about not drinking and driving, but are we saying enough about just not drinking period. When parents do find out the concern and punishment is much different than if you find out your kid was doing a hard core drug. And the kid's look at it much differently too.
At any rate, I still think it is the forkin' cell phones that are the biggest problem with teens and driving. Even in this accident, at 10:04, seconds before the accident, Bailey, the driver of the SUV that all the girls died in, called the other's in the car following saying she was passing the van, but wanted them to stay together on the road. She probably still had the phone in her hand, perhaps was in the process of calling back to tell them not to pass yet - a truck was coming, when she veered over. And think about music playing (one had just made a new mix CD they were excited to listen to), four other girls with four other cell phones, all that conversation and distraction, that driver had too much going on. That would be too much for a 40 year old.