It's also possible that one person's "low-grade bullying" is another's "gee, I'm glad my bullying is not that bad".
Again, bullshit. People with severe OCD, bipolar, schizophrenia, etc. can clearly have little or no control of their emotions and reactions. Abused children deprived of social interaction can lack the ability to even develop language. For example, http://www.tampabay.com/specials/2008/reports/danielle/
Verbal abuse is recognized as a phenomenon in medicine, religion, and in law. It is not at the same level of unacceptable civilized behavior as physical violence, but it is still unacceptable nevertheless, and can carry severe consequences, quite rightly.
Many people are actually not deeply self-aware or in control of their emotions or reactions. This often is due to the environment they grew up in, their age/maturity, and can be overcome. It also may be due to illness that requires treatment. Some are victims of abuse, that hurts their career, enjoyment of life, etc. Yet abusers themselves are often the most egregiously acting without self awareness or emotional control, as they are often lashing out what they're feeling without restraint or contextual nuance, attempting to inflict pain on others.
Yet other people also are deeply self aware or in control of their emotions but due to community or societal norms are subject to systematic verbal abuse. Sexual harassment, threats against geeks/nerds, racial discrimination are all examples. There's often only so much any well-adjusted person can take.
You seem to be confident that words should not be able to inflict pain. Yet it's clear from history that they do.