It may seem harsh, but there are rules when players are suspended. Legia broke those rules, clearly through human error as I understand it, so the club must suffer the consequences. My husband was secretary of a non league club for 5 years, so I'm aware there is intense pressure to ensure the "paperwork" is correct and inform the manager which players are ineligible for certain games. I see similar offences in the past have been punished differently, so there is inconsistency. As for Celtic, the 3-0 arbitrary scoreline effectively eliminates Legia on away goals, so that's presumably the reason for that. Most seasons in the FA Cup you will see some non league side thrown out for fielding an ineligible player and the "losing" side re-instated.