Many nonviolent crimes are commonly associated with potential for violence. For example, selling meth is not a violent act, but betting that a meth dealer is a level-headed pacifist is not a very safe wager. (I am not really agreeing with the indiscriminate use of violence out there — just explaining the most logical rationale I've heard.)
And that's why all police officers are already armed with pistols.
A no-knock warrant with holstered pistols would be entirely reasonable in a residential, non-violent crime conviction, don't you think? (Well, I would debate the no-knock part :)