It's not that it's really that huge of a deal, just that it's so out of character for Java and everything it is seen as.
It's like if a guy you were friends with since college and always knew to be solid and reliable but a little boring, and now many years later, he's happily married with kids, watching sports, working in a large hierarchical organization, doing pretty average stuff but nothing unusual or exciting. If you suddenly found out that guy had secretly been a Furry the whole time, that would be really shocking.
Not that there's anything wrong with being boring and reliable, or being a furry. But the sudden change in how you saw somebody or something is stunning.
I half expect anything written in Java to be littered with AbstractFactoryFactories and giant frameworks using 20 different design patterns to write Hello World. I never would have expected Java to silently convert an actual null to the string "null". I always thought the one thing you can count on Java for was to be strictly strongly typed, and never silently do any weird random conversions that nobody would have expected. Guess I was wrong.
Though poking around in a few other languages, JavaScript does indeed do that also, though I kinda expect JavaScript to do things like that. In Ruby, nil.to_s gives empty string, and adding a nil to a string gives you a type error. In Python, trying to add None to a string also gives you a type error, but str(None) does give you 'None'. That's a bit disappointing, but not shocking to me.