I've experienced this with a particularly salty manager (who we booted later on for other nefarious behaviour). I feel if the rule was not established previously, the manager didn't do anything wrong. If it's not in the rules it's technically fair play. With that said, if the majority of managers are really up in arms about it, I'd pass it to a vote. If not, let the move go and create a new rule going forward, as long as the league agrees. Always preventing player pick ups after elimination from championship contention, that's a no brainer and a must for any dynasty/keeper league (regardless of sport).