The owners of the limo/party bus need to have clear rules and policy in place and most don't. A vehicle on a public street is considered a public place as in "Drinking in public".
Many companies pay lip service to this but don't encourage the drivers to enforce it because ignoring it is a big money maker for them.
Personally, I don't have that much issue with adults drinking on a party bus. I do have an issue with the under-age doing it without any discouragement.
Of course, much of the problem stems from deeper issues such as peer pressure, poor parental examples etc.
I will concede to Hatrick though that them driving is even worse.
It is illegal to drink in vehicles, as they are considered public places. However, most limo and party bus operators, including my own, do not follow the rules. Limos are set up with glasses, gobblets, and ice coolers, thereby condoning, if not encouraging drinking.
Our company leaves it up to the driver whether or not he/she will allow drinking. Most of us just keep a closed eye to it and hope we don't get caught.
The party bus is a bit different. It is advertised as a "party" bus. Those riding them EXPECT to drink. I have no direct knowledge of what kind of instructions their drivers are given.
I, for one, HATE driving a stretch limo when passengers are drinking. I have only had one passenger get unruly. But, we are the ones who clean the vehicles after the trip, and they are often left in a mess with bottles, and junk left behind.