I think the fact the use and abuse of legal opiates are much more common (abuse is estimated an 10% of US population each year) than heroin (use is estimate at only 1.6% of US population even once during their entire lifetime) explains this.
Heroin is made much more dangerous because it is illegal and therefore of unknown potency and purity.
(Hate to be that guy, but:) Not ‘legal’, but licensed opiates that are abused, beyond or outside of a prescription — and as such can often fall under the law. Some of those can have the appearance of licensed drugs but be imported without control or be replicas. I’d talk about “opiates under their licensed forms”.
Doesn't surprise me. While being a heroin addict requires knowledge of risks (dealers don't like customers dying on them), popping a few OxyContin that you have left over from surgery doesn't, despite being nearly as strong.
Heroin is made much more dangerous because it is illegal and therefore of unknown potency and purity.