DC, Cooper said ER, right? NO ONE is suggesting the use of A by some in the black community justifies the use of white folk saying ER. Additionally, any white person who is cultured understands that the use of A by some in the black community doesn't justify the use of A by white people.
And to answer your question about why do white people want to say it... 99% of us don't. Those that want to, do (in public, in front of black people, and I've never seen them get checked for it, which always surprises me).