No-one is saying motorists are justified in getting angry at cyclists who innapropriately cycle in primary.
Afaicu what is being said is ;
It may cause motorists to become unjustifiably annoyed/angry - this in extreme cases may result in possibly life threateningly dangerous consequences for the cyclist in question.
Cycling innapropriately is selfish and by definition wrong.
Everybody agrees with this.
But as I said before, most motorists won't know when a cyclist taking primary is doing so appropriately or not, and some will get angry no matter what the cyclist is doing. Let's face it, many motorists get angry at the drop of a hat.