Sanctuary cities protect illegal citizens of other countries then why?

Why can a city refuse federal law, yet a citizen do the same and its considered aiding and abetting a fugitive of the law. Punishable by fines and jail time. If democrats refuse to follow law what keeps 2nd amendment laws from being refused as protest in States like California where they disregard republican voters. A bigger question is how would democrats deal with republican voters disregarding their anti-right laws if they were armed with fully auto weapons? As its a federal law to not have that what's the difference in refusing one federal law compared to the other. In your opinion what would they do and how would they deal with military grade armed citizens? Would it get spotlight or be covered up? Please provide a thought on the matters in a detail manner without insulting.