I'm unfamiliar with the laws and jurisprudence on bench trials in the US, but the impression I have is that judges are either required to simply follow the law, or are at least bound by custom to do so.
Most of the time a judge doesn't need to provide justification of any kind when issuing a verdict. The assumption (and it's a big one) is that they follow the law when issuing verdicts - like you said it's more like a custom. I suppose the difference is that a judge cannot stand up and say "I found you have broken the law but I disagree with the law so I won't find you guilty" while a jury can - but judges have an almost absolute discretion at deciding what the verdict is, if they internally disagree with a law they can just find you not guilty if they so wish.
I'm unfamiliar with the laws and jurisprudence on bench trials in the US, but the impression I have is that judges are either required to simply follow the law, or are at least bound by custom to do so.