If I were to venture a guess: What is said is that it can be pollutant when in high concentrations distributed throughout the atmosphere, not that it's always a pollutant.
By that rationale, most life appeared and thrived on Earth with far higher atmospheric 'pollution' than today.
the Ordovician ice age had 10 x todays 'atmospheric pollution'
plants thrive on increased 'harmful atmospheric pollution'.
They had to perform the research, yes. They had to come up with some kind of consensus as to whether CO2 was a major contributing factor, yes. But, you claimed that they "had to claim it as a harmful polutant", which is not the case. They found that CO2 was harmful. But, to claim that their decision was influenced by anything other than the evidence is nothing more than speculation.
Administrator signed two distinct findings regarding greenhouse gases under section 202(a) of the Clean Air Act
this action was a prerequisite for implementing greenhouse gas emissions standards for vehicles
by their own admission, the motivation for the producing that finding was being able to leverage the clean air act, that's beyond dispute
whether or not they really believe a couple extra molecules CO2 in 10,000 of air is harmful- is another matter, environmentalists are not exactly known for their scientific literacy so I think it's possible some of them actually believe it yes.