Distraction to the student
The evidence that classroom use of technology for personal reasons distracts students is sizeable. The question is, how can teachers get students to put away their phones and focus on learning? Even with a policy and overt attempts to enforce it (confiscating the devices, interrupting class to accost the offenders, etc.), without constant surveillance from various points in the classroom, it is very, very difficult to ensure that students are not using their devices. If the class is large, it is all but impossible. And that kind of vigilant enforcement is not without costs. If the teacher must be constantly monitoring who’s doing what in the classroom, that distracts the teacher just as effectively as the technology distracts the students.
Students and their devices have become virtually inseparable. They (and some of the rest of us) are using them constantly and find it difficult to disconnect for any amount of time. Couple that problem with the fact that most students are pretty strongly convinced they can text or be online, and do other things (like drive, carry on conversations, eat, and take notes in class) without the technology affecting their performance of that second activity.