The way they see it, being white means nothing. So a character's race being white basically means blank. Only by adding a "real" race to them do they become a real character. This is why making Spiderman black is acceptable because it is seen as improving the character, but making a black character white is seen as taking something away from the character.
Why, yes, that is retarded.
I think another part of it is laziness. It's easier to demand that a creator change their hard work to suit you than to go out and create something for yourself. These people want to be catered to, which is hugely hypocritical for a group people who constantly accuse others of being entitled. For example, they want more black superheroes. Fine. There's nothing wrong with that, but they don't want to create new characters with new stories and histories, they just want to take something that is already established and change it. They don't want something new, they want the old changed to suit them and if anyone complains they get called racists/sexists/homophobes etc...
Discussing my own experiences for a moment: I used to write a lot of fanfiction when I was younger but eventually I reached the point where I had to change the characters so much to fit into my story that they were barely connected to the original characters. That's when I realised I had to stop writing fanfiction because it was both a creative limitation and I had the respect for the characters to realise that what I was doing was wrong. I was taking someone else's work and bastardising for own usage. But what the people pushing for Black Spiderman are doing is demanding that the comic be changed to suit their fanfiction. I think the MarySue would have called it cultural appropriation but that's melodramatic bullshit. Sorry if this got off-topic.
