Because students could use their mobile devices for research purposes during class, which is ten times easier then walking to a classroom which has computers just to search one question on the Internet. Teachers also have a job to tell the students when to stop them from doing something wrong. So if a student isn't searching on topic then the teacher can simply confiscate their mobile device, easy as that. Also students may feel more comfortable with their mobile devices when researching.
It's obvious it's not about the students disrespecting the teacher while on the discussion but their learning scope should go wide. What the students will learn after all is the main outcome needed for every student. Lessons were more explained online and let students use the phones while in a discussion for more proactive brainstorming and more interactive classes.
While the standard "Oh, kids learn more by using their phones in conjunction with lectures" is a nice sound bite -- it never happens that way. After multiple semesters to test the efficacy of different strategy and allowances of phone use in a classroom setting have shown (as a completely nonstatistical or scientific test) that the less a class uses a phone during the structured lecture times the higher test scores
If kids were allowed to go on phones during lesson show will the teacher know if kids are on work, not social media or playing games. Why would students need a teacher if they have their phones to go on instead, real social life wont be needed as they have their phones.