I experienced this problem as well. With only a small number of files to sync, having the app installed on my phone used 20MB of cellular data in just 2 days. It does not appear to be syncing any files using cellular data. It's probably just constantly communicating with peers, checking for new files, though it won't actually download the new files until Wi-Fi is available. Needless to say, the "don't use cellular data" setting should apply to ALL forms of traffic. My solution: until this is fixed, use the Android settings to restrict background data for the BitTorrent Sync app. Go to Settings > Data Usage > BitTorrent Sync > Restrict background data. This does not affect background data usage when connected to Wi-Fi, so BTSync will continue to work on Wi-Fi just like you want it do, but nothing will touch the cellular network (unless you leave the BTSync app open in the foreground).