Prove it.
Prove that every head coach knows EVERYTHING going on with his program at ALL times when it comes to things his assistants are doing.
See, I get what you're saying... but the "prove it" mindset is exactly why steps are taken to ensure plausible deniability. It's in the same vein as "it ain't a crime if you don't get caught".
Will a HC *always* know what's going on? Unlikely - especially if it's a one time thing, or a special situation.
Will a HC *likely* know what's going on? Yeah - especially if it's a repeated occurrence or a pattern of behavior.
A "clean HC" with "dirty assistants" may at times be similar to a mob boss - he may damn well be aware of what's going on, but he's taken measures to ensure that none of it can be tied back to him.
If a HC doesn't know what's going on with his assistants (especially if it's a pattern of behavior), there's a good chance it's due to plain incompetence (no control over or awareness of the actions of his direct subordinates).