Why not? "Assume good intentions" is really a pretty fucked up way to keep people down. There is so much scum around, assuming good intetions is bordering on dangerous naivity. So it is far safer to dump all shitbags into a single container, and stop caring about the supposed subtle differences.

Because we're actually quite bad at understanding people's motives (e.g. fundamental attribution error). Not to mention the various ways information gets biased as it flows through people and institutions.