And since the user has the original key, it'd have to be voluntary surrender. After you turn your key in, you lose access.
The best part is the whole thing could be reviewable and added to a public immutable ledger, encrypted, to make the whole process, transition, and access transparent for courts later. Wouldn't it be great if more investigations happened that way?
And if you don't trust law enforcement, thats your prerogative, no need to use the feature.
I'm pretty sure being able to access an Airtag that was put into stolen mode by the owner is the least of your concern. I'm not even sure what failure mode you are worried about because you didn't elaborate.
Please don't think I'm trying to be all high and mighty because I live in the UK and am surveilled even worse than you are (although at least our police are very rarely armed).