Forkability is necessary.
You misinterpreted me. The ability to fork, not only it is necessary, but it is inevitable in a decentralized system. I'm only saying this: Splitting a chain, besides consensus-wise, it splits in security as well. If Bitcoin Cash didn't exist, some miners wouldn't mine it, and so they'd most likely mine its past fork.
Someone could grab your signature from the other chain (which includes the block height), and post it as evidence that you submitted two votes for the same block height, resulting in your balance being slashed.
But, it can't know that I'm voting or just spending my money, unless they forbid me from using other forks, which doesn't make sense because chains are independent and don't interact. Centralized blockchains (which is a contradictory term anyway) could be coded likewise though, no doubt.