Because making an ad supported version and offering it for free is an extremely effective way to combat piracy. If if you side load the app that you downloaded from a shady mediafire link, the ads are still tied back to the developer and provide a revenue stream.
If i were an app developer i would have an ad supported free version and not offer a paid version for this very reason. If you think about it, there are far fewer people who have rooted phones blocking ads then there are people who have the ability to pirate apps because any android phone, rooted or not, can sideload apps. Only rooted phones can block ads