Quote from: NomadicLet's please get back on topic. We can argue semantics all day about this but it is getting more and more off topic. The OP was a quickly thought up example and tearing it apart for loopholes is a bit silly. :PI don't think it's entirely semantics of a quick example.
The temptation to take away some (or all) of the PCs rules-given power to pull off a certain situation is very real for the DM. In your example it was denying the PCs any Perception checks to prevent the murderer of the girl, in another example it may be delaying initiative checks by one round to make sure the PCs can't prevent the killing of innocents/trap activation/etc, while in yet another example it may be denying the PCs any Insight checks to find out the noble's lying to them.
While doing such a thing may be an easy and quick way to set up a certain situation, it's a dangerous pitfall that often comes back to bite you later.