With PoW, a miner needs to make an investment in mining equipment for to mine the cryptocurrency. If that equipment cannot be used to mine that cryptocurrency, there is no other way for the equipment to create value for its owner. If the miners as a whole were to act badly, the mining algorithm could be changed, and the mining equipment would become worthless.

Similarly, with PoS, a miner must "lock up" their coin in order to receive mining rewards. This is similar to a miner buying a miner, except without the mining manufacturer. If a PoS "miner" were to produce invalid blocks, their "locked up" stake would be lost. It would be up to the other PoS "miners" to determine if another miner is producing invalid blocks. If the miners as a whole were to act badly, the value of the coin would become worthless.

In both PoS and PoW, if the miners as a whole act badly, they will lose their investment. However, with PoW, the underlying coin will survive. This is an important distinction. If a large PoS miner were to successfully act badly, they could potentially enter into derraritive contracts to offset the coin they have "locked up" that is mining. This removes the requirement for a PoS miner to actually invest in their "mining" operation. OTOH, a PoW miner cannot easily hedge their investment if they intend to act badly while mining.