I think you are forgetting what it was like when the "black lives matter" slogan was first becoming popular. If hadn't already been exposed to it, it was very easy to respond "incorrectly".

For example, here's a case of a Democratic governor, running for president, getting caught by this:

https://www.cnn.com/2015/07/18/politics/martin-omalley-all-l...

And here is Hillary Clinton, falling into the same trap:

https://www.npr.org/sections/itsallpolitics/2015/06/24/41711...