Defoe is not intentionally anti-imperialist in Robinson Crusoe. Crusoe, his main character and first-person narrator, accepts unquestioningly the foundational assumptions of imperialism/colonialism: he assumes the right to claim ownership over and exploit any land he finds, and he assumes a "natural" racial, cultural, and religious superiority over the indigenous people who enter "his" land.
It is we as twenty-first century readers who see the immorality of imperialism in how Crusoe treats Friday. Crusoe is able to save Friday's life through his superior firepower (he has a gun) and then assumes it is his right to rename this new "subject" Friday, treat him as an inferior, use him as a slave whose will is subordinate to his own desires, and convert him to Christianity. In a nutshell, we are able to see the many impositions imperialism made on indigenous people.
Today, we understand as immoral Crusoe's assumption of superiority over another human being. We see the arrogance of Crusoe's one-sidedness in his treatment of Friday, which mirrors in microcosm the arrogance of the West towards native peoples across the globe. Crusoe has no interest in learning from Friday, because he doesn't believe someone like Friday has anything to teach him. He asks no questions about how indigenous peoples on nearby islands live or arrange their economies (though that might provide him with valuable information) and shows no curiosity about Friday's religion, language, or culture. He evaluates Friday in terms of European norms, finding Friday appealing because he has features that are more European than African or Native American: for example, his hair has a looser curl than African hair, and his skin has a tone that is more attractive, in Crusoe's eyes, than the normal native hue. Crusoe, however, never shows any awareness that he might appear repulsive to Friday or in need of correction or instruction. He simply cannot conceive that Friday would do anything but look up to him as a superior, godlike being whom it is his great fortune to serve.
A good counter-text to the original novel is Michel Tournier's post-colonial Friday, in which Friday is able to model for Crusoe a new and arguably better way of living.