I don't see why it would be. The police have no obligation to share the "why" behind their thinking at this point. They could have just said, "we're looking for

I don't see why it would be. The police have no obligation to share the "why" behind their thinking at this point. They could have just said, "we're looking for a white Elantra" and left it at that. If they lied about where the tip came from that would likely come out during the trial and hurt the prosecution's case. While some people have a moral objection to it, using DNA in the way they did is totally legal. If that was really how they got the tip, they gain little by lying about it, but getting caught in that lie would have a massive downside.