I think calling them "blatant lies" is counter productive, although I can definitely see where it can seem that way especially with Christopher Columbus. I'd say it's a combination of various biases that heavily distort the truth. One bias, as an example, would be idolization, where people have a tendancy to idolize people (like Columbus) who achieved something ("Discovering" America). Don't blame underpaid elementary or secondary teachers for not being able to fully see through these biases, it's just not something that everybody spends a lot of time thinking about.

I think it's good that we are able to talk about Christopher Columbus in a more balanced way now that some of his flaws have become more known, and I wish people would continue talking about it without blowing it out of proportion.