Dutch law distinguishes between "soft drugs," like marijuana, and "hard" ones, like cocaine and heroin. Possession and use of up to 5 grams of marijuana, and 1 gram of cocaine or heroin, is not subject to penalty. In sharp contrast to the U.S., where drug use has primarily been dealt with as a criminal justice issue (although there's some evidence this is changing), the Dutch approach emphasizes harm reduction and public health.
One of the drawbacks of a criminal justice approach is that the threat of harsh sentencing keeps many drug users from seeking medical assistance in the event of an overdose. That's not a concern in the Netherlands. The Dutch approach allows authorities to have a frank dialogue with drug users when new dangers arise, like the fake cocaine.
The destigmatization of drug use in the Netherlands also plays a big role in this. Drug users there aren't thought of as criminals, as in the U.S., but rather as normal people engaging in unhealthy behavior. There's a notable lack of moral judgment in the language used in the Amsterdam cocaine warnings -- contrast this with the rhetoric employed by many of the opponents of drug law liberalization in the U.S.