In the case of selling drugs (assuming that drugs are harmful, and so on, which is obviously debatable), then selling and delivering the drugs to users causes tangible harm that wouldn't happen without the sale of the drug.
In the case of child porn, the child porn has presumably already been created - they're not filming new child porn and distributing that, I presume. So the bulk of the harm is already done. The direct harm from pedophiles downloading that porn and viewing it is minimal. The problem with the child porn is that it needs to be filmed/created in the first place, which involves child abuse, a direct and immediate harm, and that people who enjoy child porn are asserted to eventually graduate to abusing children themselves (a more distant and hypothetical harm, which is negated by arresting those people after having identified them).
So, whilst I also dislike the police's actions in corrupting tools useful for free speech, I don't see that letting the server run for another two weeks to identify more users, and actively selling and distributing drugs for two weeks, are equivalent.