Because the real premise is just what you were guessing with about Ageha. Cecil is a lawyer to exonerate her mother, currently on death row, that might be the only reason. That little reveal also showed us that the court doesn't necessarily execute wizards right then and there, though they might for the really blatant ones.

Umetsu Yasuomi specializes in series focused on the underworld and fringe justice system, so I wouldn't say he's unfamiliar with the material. I'm hoping this episode was just a fluke due to the shitty way they decided to resolve the case based on the fact that neither side had good evidence, but the court is biased toward condemning wizards based on circumstantial evidence. Cecil herself got fined twice in this episode, and she was clearly defending herself or non-wizards companions.

It's too soon to say about Ageha, but maybe she just likes being a lawyer? She certainly surrounded herself with some very intelligent and highly skilled people. Her brother is her Partner at the firm, and she's hiring on a lot of smart but young and highly experienced wizard barristers. She also has gathered a capable and enthusiastic paralegal and a receptionist doubles as an investigator who just barely operates on the good side of the law.

During the scenes at the docks, Ageha definitely enjoys getting into trouble, and is a bit more than a pretty face. I'm hoping she's less spy and more strategist. Did not display ninja quality skills using that shoe blade...