To me, it seems like a time consuming, expensive way to go, going door to door, and as someone posted, returning for a second time if someone isn't home.Totally agree. I don't think we can rule out that they have a suspect or suspects just because there has been no arrest yet. LE can't just demand anyone they consider a suspect give a DNA sample. They need enough evidence in order to demand it. And I don't see the DNA dragnet as being contrary to them having a suspect or suspects. In fact, doing such a dragnet in a suspect's neighborhood would be a great tactic. Obviously the suspect wouldn't voluntarily give a sample. To be clear - I'm not implying anyone who refuses to give a sample could be guilty. (I feel I need to state that as we have some people on this thread that seem to misread others' comments.)
I honestly could go both ways on this - LE might be trying to gather enough evidence against a suspect to require a DNA sample or they might not have any suspects.
I think the more direct way to go would be to take the guy in for questioning, obtain his DNA either at the police station, off a drinking cup etc, or stake him out and obtain his DNA somehow that way, (ie trash, etc.)
This makes me think they have no suspect.