Nothing other than the fact that it's the slippery slope of "let me search X if you have nothing to hide". For why it's scary, read this comment just below: https://news.ycombinator.com/item?id=11683721
With enough of these in a city police etc. can track every single person in the city at little to no cost with accuracy only limited by the density of these readers. It's within spitting distance of having a GPS tracker on every car in the city 24/7.
I'd be willing to accept them if they only captured and stored information on people with warrants or under investigation but without any cause police shouldn't be able to track my every movement and store that data.