Or insist you can be racists against white today because they deserve it, then there's a good chance you would have said the same against blacks a hundred years ago. Same with Jews in Germany in the 1930s. There's no reason to believe you would have stood against it. After all, just like much of the justification for stereotyping all white Americans today, much of it was from on high back in the day. Whether the news media, theater and film, government, business - all were party to advocating racist attitudes and stereotypes. If you are embracing racism against white Americans just because those same institutions are doing so today, there's no reason to think you wouldn't have done the same back then.
The problem with history is that we have to wait for it to happen in order to study it. Likewise, it's easy to sit around and condemn the sins of the past and the people who committed them. I think that's why we spend so much time doing so nowadays. Plus it keeps our eyes off the current sins we're seeing grow around us. Sins that are merely taking seed, but haven't grown to full fruition yet.
The solution to the racism of the past is not to replace it with a new racism for the future. The same with sexism. If we feel women were treated poorly just because they were women, the solution isn't to turn and treat men the same way. All that says is that we adore sexism and racism, we just want to make sure we get the right demographics this time. That, of course, makes you no different than the sexists and racists of old, who thought the same thing.