Because it supposedly puts people on some kind of watch list? Tho pranking with stuff like that ain't exactly the coolest thing to do: "Haha I framed you for terrorism, gotcha!"
Also, Google saves all voice commands. If the NSA wants to check they'd just hear all of us doofuses giggling and trying to one up each other.
But if things like this _actually_ trigger any sort of response, then the effed up thing isn't the (childish) prank, it's the system actually acting on nonsense.
If I call the police and claim that you're armed and hold hostages, then that's irresponsible. If I shout bullshit instead and these novelty devices turn that into a web search, there's no way in hell that should have any consequences whatsoever.