This is nonesne! As no property is really being stolen, as the person is no doubt on a flat fee anyway
Still, it takes away available bandwidth and thereby has a negative impact on the throughput speed that the owner can get himself. that makes his own downloads take longer, so if nothing else you're stealing his time.
furthermore, a much more dangerous issue: what if the person 'borrowing' the internet connection uses it for illegal purposes? Hack other computers, download child porn, etc. Any evidence of that activity will point back to the IP address of the router, and the owner can end up in serious problems trying to prove it wasn't him/her doing it.