I mean, the players can make imperialist characters if they want to, the same as how you can make an asshole character in a DnD campaign.
I say that its punk because the setting and rules don't depict this as something that's good or neutral. The sourcebooks give a description of how the colonial economy operates, and some of the monstrous things that are done to maintain it.
Cyberpunk depicts a techno-integrated capitalist hellscape in the same way. They essentially take ideas to extreme conclusions to show how hellish things will get.
Steampunk's alittle different, but the satirical depiction of colonialism is pretty punk.