@Anders Have a read of this thread, while from May this year it does outline the change to non-persistent vaults being the preferred operation of the network for security reasons.
The only advantage now that a persistent vault has over a non-persistent vault is that it has more availability to serve chunks (earn) and doesn’t need refilling each period since it is never turned off. Also ranking has changed to account for the move from persistent to non-persistent vaults as the preferred vault operation.
To answer your question, its not cheating but the preferred model, and as @happybeing said the vault only earns when serving up requested chunks successfully and your rank now depends on successful GETs rather than time turned on