For the same reason it’s always been frowned upon, questioning the integrity of the referee devalues all of them. They're already scrutinised to death without public accusations of bias by the clubs themselves.
If people really thought rationally about it, if you were in a job where you knew there was a potential of an accusation of bias, and that would be publicised to millions and your every move scrutinised even without that potential bias, what would your natural instinct be? It wouldn’t be to fulfil that bias, if anything it would be the complete opposite.
People assume that everyone is a fan to the same level as they are and assume everyone else’s thought processes will be influenced as theirs are. That’s the perception Forest are amplifying in their statement.
Complain about the decisions absolutely (although I think the first two, he was right to keep with the on field judgment, the third was a shocker) and ask for an explanation for it. By doing what they’ve done though, the ultimate consequence if everyone did it and it was allowed is we’re going to end up with an even smaller pool of remotely competent referees than we have now at the top level (and yes they are competent and have to go through a shedload of assessments to get to that level in the first place).