The procedure for recruiting 'informants' is more or less a textbook implementation of "How to get unbelievably shoddy evidence, up to and including outright lies, planted evidence, wacky entrapment fun time, etc. without having to specifically request any wrongdoing"
You catch somebody doing something that you could put him away for a sufficiently scary amount of time for.
Then you suggest that his cooperation, especially his fruitful cooperation could really help in terms of what charges actually end up being sought.
You then either give him 'get us results along these general lines'(if you suspect that he is familiar with the sort of people you want dirt on, and you are just fishing) or point him toward a specific target(if you have reason for that target, or you suspect that the 'informant' is basically a noob who you are just using to do something you don't want a cop doing, these noob type informants end up dying on the job and embarassing amount of the time, but they aren't sympathetic characters so nobody does much about it.)
There you go. Unless you are a real idiot, you never tell the informant to do anything illegal(and if you do, you definitely do it verbally, 'cause who are they going to believe, Officer Hero or Scumbag the Perp?); but you've just created a very strong incentive for your hapless informant to do whatever it takes to bring in the results you request.
(Please note, I don't mean to deny the possibility that this specific PD was actually stupid and/or arrogant enough to order an evidence plant, hell, they might have provided the crack, especially if their informant was some pothead who wouldn't even know who to get the harder stuff from, and if this was the case, I hope the lot of them fry. My point is, though, that using coerced informants in this fashion is one that, overall, might as well be designed to produce such abuses without anybody ever having to give illegal instructions, or even suggestions, to the informant. This is really much more serious than a hypothetical system where it takes illegal instructions to get abusive behavior.)