I though it was just the Empire holding on to them, but reading back the Galnet articles it seems you're right.The ones rescued from the pods probably aren't the ones we need to worry about
- they're either still in quarantine (Federation, Empire) or have been released into the community but are likely still under heavy surveillance (Alliance)
- it's possible that whatever treatment the Thargoids were using hadn't been applied to them yet so they're actually perfectly safe (as in, they are no danger to us; the mob with pitchforks is a big danger to them, unfortunately)
It's the ones they captured, treated and released without us ever noticing some time in the last twelve months/ten years which you might need to watch out for.
Winters has said she wants to release them, but hasn't taken up office yet to do so. Posting the articles here as a reminder for folks given that it's on topic:
Alliance
Federation
Empire