But by law I don't feel they should be required to.
Yes but if you encourage the crime you are guilty of the crime. The video had at least two of them encourage the crime, by taunting the girl and laughing and pointing.
The U.S. criminal code makes aiding and abetting a federal crime itself a crime:
(a) Whoever aids, abets, counsels, commands, induces or procures the commission of an offense, is punishable as a principal.
Aiding and abetting an attempted murder is accessory after the fact. Anyone that did not come to the aid of the girl or at least called the cops committed the crime.
But given your example. If you had a bank robbery, and a person in the bank started to encourage the act, telling them where the cash is, not calling the police, and filming the crime, it leds to the concept of "guilty mind".
The people in the mcdonalds knew a crime was being committed, but did not want to help or get involved until the girl was seriously injured. By laughing, pointing, commenting, instructing, and commanding the girls to leave, they acted as a conspertor in the crime and there for have a guilty mind which is grounds for accessory after the fact.
Should everyone get charged, no, the person that called police when the crime first happen should not. The people that tried to stop the crime should not.
But those that stood around recording, commenting, instructing, and commanding the attack should be charge.
That is the law.
As for the law suit for mcdonalds, the assault happened on their property, so they are legally liable, that to is also the law.