It seems this thread is getting derailed a bit.
The OP's question wasn't is it moral to connect to an unsecured router, or even should it be illegal to connect to an unsecured router. The question was is it legal.
The answer, pretty much no matter how you want to swing it is no. it is not legal if you deliberately gain access to someone else's router without their permission.
How illegal it is, whether you're going to get punished, and whether the punishment is acceptable to you is an entirely different question.