There is no engineered addiction to reading the New York Times, so people just put it down when something else wants their attention.
Looking at a phone is a problem to the extent that it cuts you off from real interactions in society. It is a problem to the extent that the attention you pay to the phone does not go toward solving real problems.
It can be a problem because it allows kids to escape from uncomfortable situations like struggling to learn something, and the Instagram-perfect view of the world makes their own lives feel inferior.