I'm sure there's a reason for this but I don't know what it is.
Why aren't Inquisitor's heavy handed? For example, your Inquisitor hears a rumor that there are mutants in a hive. In the game, you'd have a great scenario involving investigation. But (1) Inquisitors have vast amounts of unchecked power and (2) most of the quotes in books and such imply that people don't count, etc. So, why not simply order that the entire lower levels of the hive be eradicated in some horrible way? It's not illegal - normal people don't matter in the 40K culture.