It could be argued that they use it precisely because of the ambiguity. The word retains an aura of taboo and evil and one of the worst things a person can be. Then you redefine it to use it against a broader set of people but continue to harvest the power of the previous connotation. A variant of the euphemism treadmill. Instead of dropping a tarnished word and transitioning to a clean fresh one to describe the same thing, you keep using the tarnished word but for a different thing.

Closely related is the "motte and bailey" tactic: depending on the situation you sometimes use the word in an milder sense (new meaning of racist, "we are all a little bit racist", it's just unconscious biases or beliefs about any difference between races), but when people aren't looking you switch definitions (old definition, worst possible thing, "some races must be subjugated and kept away from our social circles as they are subhuman and incapable of participation by their nature and will poison our bloodline").

Indeed many critics of the new social justice movements would have this as the main argument: that SJ uses equivocation in this way as the rule, not the exception.