That's not a side effect of heroin, that's a side effect of overdosing. Death is also a side effect of overdosing on acetaminophen, and alcohol. Imagine if when I went to my local wine shop they made a mistake and sold me something that was 80% abv rather than 12-14% and I drank the bottle. Or my pharmacy gave me 5g acetaminophen tablets instead of 500mg, and I took 2 of them every four hours for a few days. Or my finely ground table salt was cut with some form of rodent poison. I'd be pretty dead in those cases most likely. Most illegal substances you have absolutely no idea what you're buying or how much you're buying unless you test it yourself. Yes, word gets around but that's no use to you if you accidentally take 10x a dose, or something that is just not what you were told it was.

Legal substances also have lots of information readily available around them - if I go to my pharmacy and ask for ibuprofen, they will tell me the dosages in advance. If I go to my doctor and they put me on oxycodone, they will give me a dose that they think is safe, (and in theory, a plan to reduce my dependence on them) without judgement. If you develop an opiod addiction, your support options are much more limited.