It isn't a Roman salute. It was a French painters idea of what the Romans did (based on absolutely nothing), which was appropriated by the fascist movement of Mussolini (who only dreamt of being Romans).
It's blatantly wrong, and this is a purely fascist thing. They are the only ones in history every using the salute.
I honestly would like to know what that French painter would think about all this.
When we wonder how people did not resist the Nazis in 1930s Germany, this is how. Everyone afraid and trying not to anger them, and all the businesses silencing dissent to protect themselves.