I am against no-knock warrants in general, because of the heightened risk to all involved. We've also heard horror stories about police raiding the wrong house, or engaging with a legally armed homeowner who doesn't realize what is going on.
This was not your standard situation, as it involved a tactical team with line of sight, and some sort of situation that may have necessitated this being done then and there (reading between the lines, something else may have been going on).
They knew exactly where he was in that house, and what he was doing. Knocking, which would obviously tip him off, is far more dangerous in my view. They'd likely lose sight of him, and a lot of bad things can happen in those several seconds.
So yeah, I see "absolutely no reason" to take that risk.