>is meth, heroin, and cocaine supposed to be safe for you?
Most likely not, but definitely much safer than unregulated "bath salts". And keep in mind, "safe" is not binary, it is a spectrum. Even a lot of common household OTC drugs are not safe if taken in improper dosages or without following the instructions carefully.
And even regarding those drugs that you've mentioned, a giant chunk of the deaths and overdoses occur due to impurities and unknown mixes that are unaccounted for due to the illegal status of those substances.
For example, let's say a heroin addict keeps buying from the same dealer and knows his safe "dosage", which is N grams. The thing is, that's not the real dosage, because street heroin is mixed with tons of garbage fillers to prop up the value and make the drug cheaper to manufacture. So his real "safe" dosage could be 70% of N, with 30% being fillers. Then at some point later, he either switches dealers or his usual dealer switches suppliers, and the fillers only account for 10% of the final product now. So the addict buys his usual "safe dosage" of N grams, without being aware of decreased filler amount(dealers don't post an accurate percentage of fillers in their drugs on the package, cause why bother if it is illegal either way, and it could be that they didn't even know it themselves). Even though the addict thinks he is buying his usual "safe" dosage, in reality his dosage becomes 29% higher (0.9N grams vs. 0.7N grams), hence the overdose.
Note: numbers and percentages are absolutely made up, because i have zero idea about typical heroin dosages and percentage of fillers in those. But my point should still be valid, even with the real life numbers probably being much different.