They earn plenty of money on Transmit for Mac.

The following may be a bit hard to argue but let me try. The availability of Transmit for iOS should increase sales for Mac, since the apps can have synergies that make the Mac app better. So I think they should credit some of their Mac earnings to iOS, and factor that into their figure for the iOS app. Doing separate accounting for paired apps on different platforms makes less sense when those apps have synergy with each other.