I don't think confronting officers, on or off duty, would have done any favors. I personally would have pulled out a phone, recorded (more than just the cars, I would have tried to at least get more information somehow), and allowed the video to go viral. I know this sounds harsh and detrimental, but at the same time if they were in fact on duty and being so negligent of their job to patrol, then yes- their crime needed to have light shed with evidence.
On the other hand, they very well could have been off duty. Even so, confronting officers about their "personal business" I would think could cause harassment issues for the person asking the questions. After all, judges tend to favor with officers, and not all officers are understanding to people who question their actions. More like a darned if you do and darned if you don't situation.
Honestly though, I wouldn't be surprised if you were right. I am in no way saying all Police Departments are full of these types of officers, but some do have the few "buddies" who find ways to "cover each other" so they can be negligent to their job. I am not saying they couldn't enjoy the game but lets face it: People everywhere have to work during big games. Does the whole department get to take a break together to watch while their bosses pay for machines/cash registers not running?
As someone has already mentioned, there is lacking needed information to know for sure what the exact situation was.