When you start using these apps there's generally a popup like this one:
When you use voice activation commands like
"OK Google" or touching a microphone icon, a
recording of the next thing you say, plus a
few seconds before, may be used and stored
by Google and associated with your Google
Account to help recognize your voice and
improve speech recognition. [1]
Wiretapping is when someone records you secretly, so I don't see how the laws would apply here. It's just like you can record your phone calls if you answer them with "hi, this is SilasX, and this call is being recorded."
[1] http://cdn3.pcadvisor.co.uk/cmsdata/features/3535224/OK_Goog...