probably setting myself up to get flamed but here goes;
a capped roster is not uncommon in many sports and prevents teams from bringing in guests to improve fortunes if results are not going too well, seems fair providing the cap is 15-18 players allowing the teams in question to have in effect 2 squads. (contractural obligations aside)
non participation at an event creates a scheduling problem and affects qualification to the ko stages if the prelim groups are not of equal sizes, although i see the point that compulsory attendance is unfair on 3 teams not having that rule is surely unfair on all the other CL teams
purely a view expressed from the outside. flame away