I tend to rankle when people compare the colonialism of the last few centuries with the pre-capitalist expansion and settlement of ancient societies. It seems like there's a lot of daylight between the English founding Jamestown and ancient Ionians founding Massalia or w/e.
But what do Hexbear's historians think? Is it fundamentally the same social phenomenon across time or is capitalist settler-colonialism its own unique thing?
Why does it bother you when people compare them?
Bother is the wrong word. I think it's easy for people to look at capitalism and settler-colonialism and see the parrallels to earlier forms of social organization. Pointing out that much of the inequality of English fuedalism survived into capitalism, for example.
But I think it's important to point out that capitalism does have fundamental differences from feudalism, early command economies, etc. Otherwise you get things like British and French people justifying Sykes-Picot in by comparing it to the Roman conquests of Gaul, or white midwesterners making apologism for Manifest Destiny by pointing out the history of conflict among ancient plains peoples.
It sounds like what's actually bugging you is the assumption that anything the ancients did was fine.
It's perfectly plausible that the ancient Mediterranean colonies were just as brutal as age of sail European colonies, but the voices of the victims were lost to pre-history.
In my experience it's almost always to try to justify modern colonialism. I'm sure lots of academics and theorists make legitimate comparisons, but in the wild I've mostly encountered it in the context of people saying "People have always done this so we're not doing anything wrong".