The Mobile Passport app serves as a replacement for the form that US Citizens fill out as they reenter the USA. If you don't use the app, you use a kiosk at the major ports of reentry to accomplish the same task.

https://www.cbp.gov/travel/us-citizens/apc

People entering the USA that are not citizens fill out a different set of immigration and customs forms, because their entry process is different.

Because only US citizens have US passports. Just a wild guess.