Probably they use a different ads network. Admob is very strict about this type of apps. We’ve seen many users have their Admob accounts suspended and/or deleted because they used it this way.
My advice: Don’t do it. Try to think of a more original idea, and put a non intrusive ad in it. In the long run it will give more results as your user base will increase and your Admob account won’t be at risk of being closed. Also, as a warning: You may earn some money doing this “click and earn” apps, but the moment Google finds out, they take all the money back. I’ve seen that case too.