As I understand it, they're calling it a "freak accident," not because stingrays don't normally sting, but because their venom isn't normally fatal to an adult human.   What Steve was doing at the time of his death was not as foolhardy as some are making it out to be.   (BTW, Marlin Perkins got bitten several times by poisonous snakes and came close to dying from it a couple times.)   
Beekeepers sometimes get fatally stung too.   Experienced drivers get killed in car crashes.
This ray's barb happened to pierce Irwin's rib cage, with the sting going almost directly to the heart, from what I've read.

As the Aussie press release said, he died doing what he loved.   And probably in a less dangerous situation than many he's been in. But people do get struck by lightning, and sometimes a normally nonlethal animal encounter turns lethal.

Sad that his death came at such a young age. He'll be missed.