F-Droid's repository excludes several projects because it might cause them trouble. For example, they block forks of the Signal client that connects to Signal's servers, a violation of Signal's TOS but not F-Droid's.

Hosting projects in their repository causes work for them and they are right to exercise pragmatism in what they choose to host. There's more freedom with F-Droid than Google Play, because F-Droid allows its users to use rival repositories as well as its own.