Street­Complete "promotes or depends entirely on a non-free network service"

I downloaded Street­Complete to contribute to OSM and on https://f-droid.org/en/packages/de.westnordost.streetcomplete/ it warns you that "This app promotes or depends entirely on a non-free network service". I'm not super concerned about that and will still try out Street­Complete but i was just wondering what non-free network service this is. Does anybody here know?