There is no goal for testnet to be POW or permissionless or without a central authority.  It just exists to help the developers of the software test it.  It is not a currency, it is a testing fixture for the developers of the software.  Any time it turns out to be less good for the testing its developers intend for it, it'll get discarded and replaced with an updated test fixture.  To the extent that testnet might have some relation to those things (like having POW) at all is simply a product of the fact that it exists to test the Bitcoin code.