What is the deal with *all* heroin being fentanyl-laced nowadays?

Pretty much every news article involving heroin overdoses mentions fent, it's just assumed that that's what causes ODs. If heroin is legal in Portugal, is it still mixed with fent there? Is there anywhere you can get "full-opium" heroin anymore? How come we don't see many reports of junkies happily shooting up and getting high like we used to? I just don't get why fent has to be mixed in with heroin at all.