It's more about what you have to lose by challenging. In this case he was not offered any discount for an early guilty plea, and it's never going to be upgraded, so we lose absolutely nothing by challenging. The reason for not challenging usually is that simply the act of challenging often turns a 1 week suspension into 2 weeks.
How bad did the strike by Dusty on DeBoer look, or Dangerfield on DeBoer? Both incapacitated DeBoer and both looked bad!
One of the worst look was Cunningham on Wilson. It took him out of play for many minutes, but he didn't leave the field (note to self in future ...). That was a full roundhouse swing like Gaff's (which "missed") and what they said they'd stamp out this year.
They had their chance to penalise that strike but didn't and now its a rabble!