It's more of a risk management issue since they do not want that one random cell phone version 9.2344191823 with a weird antenna possibly interfering with the plane. Sure it's bad form, but there isn't much you can do. If you really feel uneasy, feel free to mention it to the FA and they can tell them to shut it off.... I don't know why people have to use their devices for those 15/20 minutes on take off and landing....
There have been exceptions though. When we are stuck on the tarmac (ground stops) I don't hesitate to whip out either my broadband card or cell phone.