Colonialism is indefensible, but migration, invasion, annexation, occupation and the displacement of one people by another is a global fact of human behaviour going all the way back through our historical and archaeological record. By the mid 19th, and 20th century, it was increasingly seen as being wrong, but that understanding cannot be transferred back through millennia, or now uncomfortable facts erased. All the actions of our ancestors shaped our world, but they acted within the limits of their time. Some of their actions had a positive impact, many of their actions hadn't, but condemning or erasing on the basis of our vantage point is nonsensical.
Columbus was an Italian explorer, adventurer and chancer who wrangled finance for his expedition from Spain over 500 years ago. Colonialism wasn't on his agenda when he set sail. He was looking for trade routes that bypassed the Silk Road, and discovered the Americas by chance, but by the same chance, he also reshaped the world. If indigenous folk had killed all the newcomers and burned their ships, Europe wouldn't have been any the wiser, and all would have been different, but that's not the way history stands. Columbus is a part of American history, as is Columbus Day.
It is right to study and try to understand colonialism and expansionism. We all have different views on it, but that's how knowledge and understanding grow. No view should be silenced. No one is obliged to buy the book or read an article.
The sooner this 21st century frightened and repressive puritanism dies a natural death the better. It is a very large spanner in the works of learning and freedom.