So correct. I can say because I've been there. Not gambling, but social media. It will eat a lot of your time at work and you will never even notice the time passing by until your time at work is almost over and you are not even a little productive. What more when you combine gambling and social media at work? I bet an employee would not even do any work the whole day he is there sitting in front of the computer.
That's why the owner of the business decided to tell our computer technician to make an internet blocker that would stop us from doing it because he saw how low the production was. The technician was successful at blocking it and the owner was thankful that he saw some changes in our production.
This accounts for the reason some jobs do not permit the use of personal cell phones during work. Apart from gambling as you have noted, mobile phones can be a big distractor during work, social media, calls, the SMS are capable of distracting us from our job and thus affect productivity and safety depending on the job. I came across a story on Facebook. The narrator of the story was telling how he lost his job. He was a prison guard and they have a policy of not using personal mobile phones at work sometimes he sneaked in his phone to work and hid to make calls but he was caught and that led to his dismissal.