Why can people who clearly do not have a black complexion identify as black in the U.S. and U.K.?

There have been several news stories of white people faking that they were black. I read today about some activist in Indianapolis.  From my experience, in the U.S. and U.K. all you have to do is say you are black and in most cases, people will accept you as black.

There are even very light skinned "black" people and white people who have significant black ancestry, who identify as black when their skin isn't even brown.  And many of these people would NOT be visibly identified by others as black. 

On tv there was a lady who looked like a white woman who was complaining about being discriminated as a black woman.  Really?  How would anyone know she was a "black" woman as she claims if she does not have brown skin or African features? 

There were stories of this "black internet hacker" from the UK, a very young and good looking guy.  He was all over the news. But when I saw his white skin, red hair and freckles I thought, how could they say this guy is black?  

In most non-English speaking countries it does not work that way.  Your race goes by your skin color.  In Brazil, for example, you can have siblings who are different races because they count your race according to your skin color.  

I am 28% black genetically but I look like Scottish.  I don't go around saying that I am black, because my skin is white.  I look at lot like that internet hacker except that my hair is straighter.  I am not embarrassed by my black ancestry but how could I identify as black?