You need to read what you agree to. Some of the games require real money for virtual money. As I stated before. But, you can watch an ad to get currently in the game instead of spending real doe for it.
The TapJoy games I have played, it was in the agreement, and the ads don't play unless you activate them. It does make sense. If TapJoy is attached to 10 different games, why spend the bandwidth and Cell phone carrier mobile data (remember, not everyone had unlimited) to watch the same ad 10 times to get the in game currently for all 10 games. TapJoy just runs it from the cache.
While I personally don't like it, it really is not that big of a deal. Play the stupid video ad with the phone muted while watching TV. You get your currency without really watching it.
