why is it tied to the google account and not the avast account?!
Because avast is not a mail provider. Just because you don't know/understand things, it doesn't mean it is BS.

You ordered the license with that email account, so yes the license and email account are tied.
That makes perfectly sense.

But I have good news…
From early 2016 (perhaps even January already), avast mobile will be free (including the extra’s the premium version has now)
Your wife and son can already install the beta for it through the google store.