Tiny Tower caches video ads which can be watched for bux. If you are not watching the free bux offers, you can disable the video ads by going to the in-game menu under Settings and uncheck Video Offers.
Alternatively, if you want to see the free bux offers, you can restrict the app from using your cellular data so that ads will only be offered when you are connected to wifi. To do this, go into your main device settings and turn off cellular data for Tiny Tower. On an iOS device this is under Settings> Cellular and scroll down to Tiny Tower and toggle the green switch.