Abortion was probably not the best example, "racism is wrong" is a clearer norm that depends on words. I agree with you, as I said later on:

Schelling fences probably play a bigger role for "justifiable killing" (like self-defense, the death penalty, euthanasia), where having a strong norm against killing in general discourages revenge killings (anti-abortion seem to be trying to hijack that norm to cover a case that doesn't fall under "killing" nearly as naturally).

For racism, you get the same problem you mention, with people trying to add more things under the "racist" umbrella (such as affirmative action, or opposition to affirmative action), at the risk of weakening the norm.