Refusing to cooperate or limiting cooperation is not the same as obstruction.Sanctuary cities nullify and obstruct the enforcement of federal law, a crime
"Obstruction" implies refusal to perform a required duty.
It's a bit rich to compare perpetuation of slavery with not cooperating with those who would round up people into concentration camps.Nullification was a factor in the civil war. Southern states simply ignored federal law, and hindered it's enforcement.
What incentives?In the case of illegal aliens, they are encouraging the breaking of US law, by creating incentives and encouragement for that law breaking, a crime.
I disagree, but your point is irrelevant.Statistically, their policy has proven to encourage crime, and thus making their citizens less safe. Failure to provide protection to their citizens, there primary duty.
Any prioritization approach for law enforcement will make it easier or harder to break the law in certain areas. A police agency that reduces the resources it expends on minor immigration offenses will have more resources available for all their other enforcement activities... just as, say, a police agency that prioritizes drugs and violent crime will have fewer resources available for, say, fraud and traffic enforcement.
Deciding on priorities is not only normal, it's absolutely required.