The fact that orcs are imaginary really isn't relevant. All characters in fiction are imaginary! If that defense held water, then you could always do whatever you wanted with any fictional representations without reproach, but that is empirically not the case. Symbols matter to real humans. You'd have to go much further, and establish that there's a clear line of "imaginary enough", that magically makes it okay. Good luck with that.
The argument that fictional depictions in the media (edit: effect attitude) is Cultivation Theory and there are some problems with it. In fact, a lot of sociological science has of late had some issues.
It is currently undergoing what is known as 'Replication Crisis', or the fact many studies in the past that have shaped popular psychological/sociological myths/beliefs are proving impossible to replicate under more stringent conditions.
Might I suggest "The Quick Fix" by Jesse Singal for a more in depth discussion and review of the subject.