Allowing Pioneers to mine Pi cryptocurrency for free is a conscious network choice in service of ensuring accessibility to cryptocurrency to people regardless of their financial background. In fact, 65% of the entire Pi supply will be available for mining by Pioneers for free, which is a substantial share for the community in the blockchain space.
Pi Network does NOT use your phone’s resources – hardware, network, or anything else. The main Pi app and the Pi Browser app uses as much phone battery just as any other regular app. Please refer to the above answer to understand why Pi mining does not consume your phone resources.