I don't understand your point at all about "tainting evidence" and how that could possibly follow from brusque questioning.
Once you start deferring to police officers' immediate judgement, the legal process is tainted to amplify every single stereotype police officers hold.
White victims will obviously be treated with more deference than blacks, Spanish-speaking people will all be assumed to be illegal immigrants etc.
So the young man accused of something will be "brusquely questioned" within the first minutes and confess to the crime. While the old woman is treated with deference, has a chance to talk to a lawyer, and never gets convicted.
And the worst thing is the spiral, self-amplifying nature: start like that, and the sort of people already suspect will be the only ones actually being charged with crimes and convicted, which will reinforce all the ideas the process started out with.
You'd better believe it matters when they come after you for (say) making false statements to the police, and it's their word against yours, and the court always sides with the cops.