Why is this a problem that needs solving? If the user chooses an ineffective word they'll figure it out pretty quickly... and if they choose an offensive one-- that's on them.
They could also walk around saying offensive words when not addressing the computer.
It wouldn't be a problem if all people were decent and rational.
Unfortunately some would reprogram other people's devices to cause offence for a jape and some will get offended and blame the company for letting it happen.
The difference from Google/Amazon/Apple/MS's point of view is that if I run around yelling the N word and their device doesn't even notice then they can't be seen to be complicit by unreasonable people. It would just be me being an arse.
Heck, some would get offended at the possibility of one of a company's devices responding to loaded words like that even if it never actually happened.
Of course there is another reason: having people use the same word re-enforces brand recognition even if the wake word is not the actual brand just a word/name people associate with it.
They could also walk around saying offensive words when not addressing the computer.
Or is google planning on 'fixing' that too?
:)