Yeah, but orphaning a single PoS block with 2 PoW blocks actually costs you something (you have to be lucky or have majority of the network's hashrate).
It seems that there's currently a pool in a position to make it happen on a frequent ongoing basis. Orphaning a PoS block with 2 PoW blocks doesn't really cost you anything
extra if you're already PoW mining anyway and happen to hit 2 consecutive PoW blocks.
OTOH you can generate PoS blocks essentially for free.
Well, you do have to hold YAC, and hold it long enough, so I'm not sure it's correct to say minting PoS blocks is free.
Also, the gain from orphaning PoS is ZERO.
The gain in the miner's held YAC from orphaning PoS blocks is zero, but there may be other motivations other than an immediate gain in YAC to orphan PoS blocks.