You and I have different interpretations of "serious" in this context.
People that take the game to0 seriously are people that get livid enough about a possible game loss that they are willing to try crippling a ref to get revenge for it. The win is everything, and losing is seen as something that not only embarrasses the players, but also the town they are from.
Yes, if they took the game professionally, then they should never have tried to hurt the ref, but from what I understand the damage was done, and the team already knew they were going to lose, so they took out the stress on the ref that made the call that, they feel, brought them the loss. Again, this assumes that is the case, since a lot of the info seems to be jumping between "They did it as revenge for a call" and "They did it because he was a racist jerk", so we will have to see in the end where the needle finally drops.