In this world of cyber-communication, there are plenty of people who type first and think afterwards - President Trump being among the biggest offenders.
In communication, especially written communication which is open to scrutiny and analysis in a way that the spoken word is not - unless recorded - context is everything.
If you re-tweet a racist comment, then you are inferring that you agree with it. If you re-tweet it as a quote which you then condemn, then your antipathy is clear.
It is these simple rules which regular users of social media need to be aware of - and the higher their profile, and therefore the greater attention paid to social media observations - the more aware and more careful people need to be.
I don't do Twitter, and I am happy enough to elaborate on the odd misunderstanding that crops up on here, and I accept it as part of the downside of written-only communication.
But I can do that, because I am not a politician, and I am not the President.