Only in a police state can they expect the killer to show up for the test, or all but the killer to show.
That’s the whole point. It narrows the pool of suspects. While the owners have every right to refuse, the cops also have every right to ask for voluntary co-operation.
For myself, I’m conflicted about what I would do if placed in this situation. You want to see a killer brought to justice, but why should owning a gun make you a suspect?