Glad I don't live in the States!
The company 'owns' me 9-5, not my entire life. If they want that, they need to pay for it... and I don't see many 6 figure salaries out there...
Look at it this way; the behaviour exhibited by the invited guests of the LW was enough to offend his boss and co-workers. By not actively preventing or attempting to curtail said behaviour, it rightly or wrongly gives the appearance that the LW condones it. If I was also attending the same function, I would be upset at the LW for allowing his guests to be that out of control. So, I think it's reasonable to assume many of his coworkers were as well.
Now, from the point of view of the boss, you have an entire work team all offended by the actions of one person, who is a new hire and presumably still in their probation period. Even though his actions occurred outside of the workplace, they directly impact the atmosphere, team cohesiveness, and therefore productivity of your workgroup.
It's not a question of the company "owning" the LW when they are not being paid. It's a question of not poisoning your relationships