I didn't know that, are you sure? Sorry, I know you mention that you've discussed this in the past, but could you briefly bring me up to speed?
I have a hard time imagining that they are not obligated to enforce the laws against murder, assault, rape, reckless endangerment, and other physical forms of harm. If they aren't then I don't really see the value in having them at all. That is the only really good justification for law enforcement, to keep us safe. Are you certain they aren't legally obligated to try and prevent one of those more serious crimes in progress?
"To Serve and Protect" is the motto of most police forces, it would be a shame if they weren't required to do either. If you are correct then this is an eye opener for me.