Apple and Google Maps have habituated many of us to using satellite maps free of charge. Third party apps (such as TPE) can also use those maps free of charge in mobile apps, but only as long as you stick to using the Apple or Google provided SDK (software development kit).
TPE 3D builds its own 3D model using digital elevation data and then displays maps on top. To do that requires direct access to the map tiles, outside of any SDK, meaning that Apple and Google maps are unavailable for us to use free of charge.
There are alternative suppliers of satellite map data, but to use it, you have to pay, and the costs are not insignificant.
We hope to offer a satellite map option in TPE 3D in the future, but this is likely to require a subscription to use, due to the ongoing nature of the costs associated with provision of the map tile data.
We appreciate your understanding.