Your average junior enlistedman today has immense responsibilities ranging from defense systems to intelligence gathering. Imagine if Bradley Manning wasn't in-league with Wikilinks, but was rather captured by the Iraqi insurgency or Al-Qaeda.

Furthermore, it's an easy way to kill more Humans who would give their lives to save their brothers and sisters left behind.

: Is it for a future prisoner exchange? That seems to fall into a traditional
: war mindset that I don't think the Covenant abide by.

I don't think the Covenant imagined the UNSC having any capabilities to warrant them maintaining mass prisoner camps. The Hierarchs were only under serious threat when the Human homeworld was sided. Besides, they would've ended up killing them all anyways if the Spartans had managed to capture Truth, Mercy, or Regret whether as penance or to be huge dicks.

The Sangheili only uphold honor within their ranks. That doesn't apply to their alien comrades, and it surely wouldn't apply to our fictional defendants either.