Some people thought that was a bad idea and kept running the unpatched version. That caused the blockchain to split into two separate blockchains, the patched (ETH) and unpatched (ETC).
After that, it was up to the market to determine which version was more valuable. So far ETH is worth a lot more, so apparently the market was fine with this action in this particular case.
If people were to roll out a patch that committed a theft instead of repairing one, it's unlikely that many other people would go along with it. There'd be another split, the thieving chain would drop to a low value, and the original chain would keep rolling along with funds unstolen.