It is sort of a trick , like Yurka mentioned you have to solve the mellowads captcha to land on the cashout page. My faucets do not have a withdrawal threshold, minimum is 1 satoshi so if a user wants to withdraw right after claiming he has to solve the mellowads captcha and withdraw. The main purpose of url shortener is quite simple, not revenue (it is just around 0.0003/max 0.0005), but to extend withdrawal period without setting a withdrawal threshold for users who want to withdraw instantly. I have noticed that after adding mellow ads url most of the users withdraw once they have 1500+ satoshis.