I think the answer is (well, could be) pretty simple:
Old PAL TVs won't support 60Hz (it will result in a flickering, unwatchable picture). RF, however, CANNOT be displayed in 60Hz on a PAL TV in ANY CASE, it will automatically be reduced to 50Hz, showing a clear picture on these old TVs.
The one who performed this mod most likely didn't have a TV that supported 60Hz and therefore, wanted to use RF instead of AV.