but a forger could accomplish the same thing by choosing to forge empty blocks; thus avoiding the penalty. Its a fairly weak criticism, IMO.
So if the forger forges a block with no txs what *exactly* is the problem?
Without some sort of mathematical proof I don't really see what we are talking about (and you could make the same point about PoW systems also as they could mine a block with 0 txs as has occurred many times before with Bitcoin).