Not sure what is meant by a "default router". AFAIK you can plug in any router you like without committing a crime, but if somebody believes that they suffered as a result of you choosing an unsecured router they might have grounds to take a civil case against you.

As far as I can tell in my googling none of these negligence claims so far have been successful but there has been no clear judgement on this matter to be sure one way or another what might happen in future cases. Also bare in mind that these judgements might differ between jurisdictions.

I simply think that saying "open up your wifi, now you're no longer liable for anything that happens on your internet connection!" is very dangerous advice to be spreading.