*shrugs* That depends what side of the matter you come down on.
The point of incarceration is, nominally, rehabilitation — not retribution. (At least, outside of the USA, which goes out of its way to make it extra-impossible for felons to reïntegrate into society, and then charges them a fee for the "privilege", effectively forcing them to return to crime to survive)
The idea is to help the prisoners understand that what they did was wrong, show remorse for it, and put them in a position to not reöffend upon release. Even with 'life imprisonment', there is typically opportunity for parole, compassionate release, appeal… for pardons, for new evidence to prove innocence, for their actions to be decriminalised. That's the thing that makes it different from an execution, the potential for it to be reversed.
The Birdcage is, supposedly and officially, impossible to release prisoners from. Therefore, it is an execution — just a long, drawn out, cruel and unusual method of doing so. The only differences between sticking someone in the Birdcage, versus a lethal injection or electric chair, are A) how long it takes to kill them, and B) how much suffering and torture you subject them to first. And, even unofficially, it's still an execution — they just plan to use Zion as the actual executioner.
Flat-out execution would be more ethical than putting someone in the Birdcage. The only way to change that would be for someone in Government to admit that, yes, there is a way to get people out of it… at which point all of their supposed arguments for leaving the prisoners to run riot like they do would also break down.
The Birdcage is not designed to rehabilitate prisoners. It's designed for the complete opposite of that — to punish and torture prisoners for as long as possible. Stick 'em in a proper prison instead, give them therapy for their issues, guidance on how to use their powers constructively instead of destructively, and eventually they may be ready… to use a loaded and controversial phrase… to "pay their debt to society". "Birdcage" and "execution" are not the only options, no matter how much Cauldron like to try and convince everyone of that.