How is that worse than "they found pot under the floorboards"?
Worst case they get time off with pay while and internal investigation by their friends determine that procedures are followed.
You are trying to come up with technical solutions (oh install sewer traps). The problem is not technical. It legal, and regulatory. There is no, 0, nada, zilch repercussion for just throwing a flash grenade in barge in. Why install sewer traps and spends all that money? That is irrational from their point of view.
The floor boards and all that sewer stuff is just noise. All those "problems" are just a list hurdles they will never and or even think about solving, that was the main point.
However, I am just gathering facts to figure out what other options are available to police. If certain states, as a citizen I am judged very harshly if I don't exhaust all options before using lethal force, even if it's happening in the heat of the moment. I want the cops to do the same consideration of their other options, at least when it's an operation they get to plan out ahead of time to happen under the time of their choosing.
Maybe they bust you with less, but they'll still bust you.
Does this apply in criminal trials? If there's evidence I was flushing drugs down the toilet (and such evidence would be there), can the state be allowed to assume I had a lot more of the drug than I really did? What are the policy implications?
Edit: also clogging, not sure when this happens and not really willing to experiment.
You can't clog a toilet by flushing cocaine.
At least, you can't afford to.
That would be an interesting use for all those seized drugs. I wonder if they do this.
You could also just rig an external reservoir (or have a full bucket lying around) to rapidly refill the back tank. Though that would of course be fairly incriminating.
A sane person would probably argue that you don't get to run roughshod over people's freedoms just because a particular law is hard to enforce otherwise. But I think that's where it's coming from. You get a pervasive environment where the guys at the top are trying to be tough on drugs and the guys at the bottom keep having to let criminals go because they flushed the evidence, and it's only natural that they'll start to push for more draconian measures.