Lord Oberon said:
I don't really see how that can be abused big time by alts.
1. Create a dozen of alts, level them up, receive Sapphires from tutorial quests.
2. Accept your alts to your league, deposit their Sapphires to the bank, kick your alts from the league
3. Repeat till the desired result is achieved.
This sounds like a time-consuming scheme but you won't believe what some players do in attempts to gain an unfair advantage.
I'm not saying that everyone is like this, but we'd rather not risk creating an environment where such stunts would be possible.
I can see how alt abuse could become a problem, but it seems there may be ways to reduce/prevent it without treating all players like criminals. That is essentially what you're doing, after all. You are saying that because a minority cannot be trusted, all of us must pay the price. For what its worth, here is my suggestion:
There is already a mechanic blocking new members of the league from gaining league rewards for a set amount of time. You could incorporate something similar for making contributions to a league bank. Anyone donating would have to be a member of the league for 3 months. Considering the limited league roster, that would drastically limit/slow that particular form of cheating.
If that is not a good enough control for the devs, then I would suggest what another poster already said: Make donation to league bank an option you can do only when purchasing sapphires. That would guarantee that every sapphire going into the league bank was paid for with real money.