"Because she's harmless without the element of surprise or stronger bad guys helping her," the filly says with a shrug. "And she's a kid. If you need a more logical reason though, then she's a normal pony with a normal lifespan, so if you just put her in a regular prison, she'll eventually die of old age and then you don't have to worry about her getting loose in, like, a thousand years and causing problems." She sniffs and adds, "Unless you don't think Equestria is up to the task of containing one regular pony for sixty or seventy years. With how dumb the guards are, that might be true."