Firstly, I don't really see any Rioter going around and saying "champion x takes skill" and even less "easy to play".
Overall your description is slightly off and some champions are in the wrong place (the latter has been pointed out occasionally).
But overall what really comes to mind is how the 2nd row is generally a lot of point and click abilities or atleast not really what you would consider skillshots (obviously not true for Karthus, but nobody would dare to call him easy to play).
Garen, Morde, Nasus, Vlad (not so much after the rework), Singed, Malph, Rammus and Nautilus are all mostly point and click. Nauti has to land his hook (although he has quite a bit of CC to help him with that, Mundo cleaver is generally often used for kiting/low CD/cost so missing it isn't too bad and Cho is similiar to Nautilus.
So overall they have a pretty easy time doing their part in dealing their damage. While it's surely rather easy to kite Nasus it doesn't take much to play Nasus - mechanically seen. Regardless of how things work out all you do is press W and Q on the target.
Another thing those champions have in common is that they are not really risky/punishing to play. Missed Nauti hook? Alright, whatever. Missed Mundo Q? Alright, not too bad. Got kited as Garen, whelp, go back and heal up.
The first row is mostly super squieshy (Illaoi, Vi and Heca are not and I wouldn't consider them to take a lot of skill) and get's punished when they fuck up (Ahri/LB probably not, but I wouldn't consider them particularly hard either). If Riven goes in and doesn't kill her target chances are that she dies.
Now with that being said I don't quite see what your problem is. What exactly is your point, it's not like you draw some kind of conclusion here..