Professional teams tend to charter larger planes for post-season travel, when compared to regular-season travel. Teams want to take office staff and other employees that might not travel during the regular season, and the press contingent is usually larger.
I seem to recall years ago that Sports Illustrated had an article on what it took to get the Dallas Cowboys from DFW to an away game. At the time, the Cowboys normally took a 727-200 or a 757, depending the flight length and the number of people besides players and coaches who were traveling. But, if the game was during the play-offs, then the AA charter was a 763 or a DC-10.
A friend of mine is a pilot with AA. He worked a Cowboys charter in 1996, flying ORD-DFW after the season openers for the Cowboys at Soldier Field. The Cowboys had won the Super Bowl the prior season, yet they looked terrible against a mediocre Bears team, whose head coach had left the Cowboys during the off-season. My friend tells me that the flight was very quiet, with very little conversation throughout the cabin.
Here's the problem with diverting. If a plane has to divert because of on-board behavior or security concerns, then you will have to call out law enforcement, including Federal agents. Law enforcement will want to question people and possibly take one or more people into custody.
My guess is that United and the Giants will handle the matter privately. We know that the Chicago Cubs reimbursed the Chicago Park District for the damage to the sod on the Grant Park fields at the victory party. So, the Giants might write a check to cover the cost of cleaning the plane and making repairs at EWR.
While there is prestige in being the charter carrier for a pro team, if the team has become a headache, and airline could decline to renew a contract.