Do they need to tip their hats, even when dealing with "high value" targets? Once you've found what you're looking for, you could easily find other damning information that could have plausibly come from somewhere else, no?
While possibly true, not every situation will lend itself to easily finding other damning information with a plausible outside source.
Just look at WW2. There were times that the Allies knew things would happen, but couldn't act on them because it would tip their hat to the Germans that there was a leak somewhere (possibly leading them to the knowledge that the Allies had cracked Enigma), and the pay-off wasn't worth the risk.
