It’s because a lot of Bernie supporters see her record on issues centered around racial equality and the impact on black communities as lacking, to say the least. No need to hash, re-hash, and triple hash the reasons. We’ve all seen the super predators video by now and read about the impacts of welfare reform and mandatory sentencing.
But imagine if it was another group, say LGBT or environmental activists flocking to a candidate whose record is demonstrably worse than their opponent’s. If 8 in 10 LGBT people were voting for Hillary, we’d say “Why, guys? She only came out in favor of gay marriage in 2013 and had a major hand in getting DOMA passed.” If 8 in 10 environmentalists backed her, we’d point to her staunch support of fracking, her major donations from oil and gas industries and speaking fees from those same groups, and highlight her flip flop on Keystone only recently, when it became apparent it wasn’t going to happen.
It has nothing to do with white people talking down to black people. It’s a discussion among progressives about which candidate to support and why. No, you don’t need to explain your vote to anyone and nobody has the right to demand you do so. But if someone genuinely asks you why because they want to understand, saying essentially “None of your business and I don’t owe you an explanation.” that doesn’t make the question go away. It doesn’t help your candidate.
If you think Hillary is the best candidate for black Americans, I don’t see how asking for your reasons in an invitation for a discussion is offensive, racist, or rude.