Much like posting on LU, or pissing off your little brother when you were a kid, you only troll for the reaction. Most of the people bagging Penrith are doing it because their fans are smug and precious and feel the need to defend all accusations against their team.
Go into any game day thread and there will be any number of people with an irrational hatred of one of the teams. Most people just shrug it off, it's just tribalism and what makes RL great. I can only assume that the Panthers fans are confused as they haven't been relevant in 15 years. (please note that whilst most teams need success to become relevant, others can gain relevance via perennial choking, cheating or having jerk players)