But this can be true in POW blockchains as well. If the user disconnects his node for some arbitrarily long period of time during which there are durable forks created, said user also has to "reconnect manually to the correct chain" and the user may have to decide for him/herself what constitutes a valid chain under contentious circumstances. The user's preferred chain may have become a minority fork, for example.
In POW the onus is on everyone who runs a node or miner to understand the consensus rules they are choosing, and to be on their toes in the event of a fork (hard or soft). I see this as no different from the POS problem described above.
Well, first of all your example makes it hard to tell if you're describing a percentage-based attack (i.e. 51% attack or 34% attack etc) or a long range attack.
In your example, how much % the currency are you claiming a thief has stolen? Are you saying you're using private keys to rewrite history that is based on staking deposits that predate recent chain synchronizations by the node who's view of the history is being compromised? I can respond better if I understand what you're trying to say.