They just don't have the right to KEEP them. They can NOT keep them without a court order. Period.
Let's talk real world.
Do you know the rate at which citizens get their firearms properly returned to them?
I've worked in various law enforcement offices, including ranges and evidence/storage rooms. I've seen everything from antique muskets up to recoilless rifles sitting there. I've talked with multiple LEOs who've admitted that they make it very difficult for people to get their lawful arms back.
My brother is a detective who is disgusted by how this is handled many times. In fact, he's considering a run for sheriff next year and this is one thing we were just discussing last week that he'd like to reform.
Sure, in theory, it's great to let the government grab all the guns they want on a whim. But in reality, we need to be MUCH more cautious than you seem to want, before grabbing someone's guns.