Denver is doing no such thing. You can't obstruct something that has no legal basis for proceeding. There is no obligation in law to detain anyone for ICE without a warrant. IN fact, it's illegal to do so. There is no obligation to let ICE into the secure area of the jail to interview people. Denver can only "obstruct" something that is supposed to happen and is prevented. In the examples you cite, Ice is seeking permission and Denver simply is declining. Obstructing is when you stand in the way of something, not when you just don't say yes to a request.

But other than that...