Originally Posted by
FBinNY
....but freehubs are harder to remove (depends on the design), so flushing is often done on the wheel and that can be messy.
Flushing freehub bodies while they are still on the hub stands a good chance of diluting and contaminating the hub bearings too. Both Shimano and Campy freehub bodies are easy enough to remove that working on them off the hub is worth the effort.