Originally Posted by Zerglinator
Which is why she joins willingly so that the Summoners see no reason to imprison her. The League would be more likely to imprison her and limit her powers if she was a rogue entity threatening the world. By joining the League, however, she can unlock her potential by facing the strongest foes, until she eventually gains enough power that she can destroy/conquer Ionia, and possibly even the League itself. That's how I see it anyways.
Also, I don't really see her getting much practice sitting alone in a floating fortress.
So she turned herself into prison so the cops didn't come and arrest her? Probably not the choice Syndra would make, seeing as she's power-hungry and defiant. Besides, the end result is the same. >.>
The fact that she's in the League means summoners have power over her. If she isn't in captivity now, why would the summoners simply let her become stronger and evolve into a threat not even they could handle? You'd think the most intelligent people in Runeterra would think about these things ahead of time.
And about the flying fortress... Yeah, how many people in the world can do that? How much magic power would that require? And how much attention would that garner from the organisation that's trying to limit magic use in order to save the world?
I can't outright say this is exactly what happened, but it's the only way her lore makes sense, as it is now.