We can't prevent them from committing crimes.

There are also legal ways they can sell your shares without your consent:

Yes, brokers like Fidelity can sell your shares without your explicit consent under certain circumstances. One common situation where this might occur is if you have a margin account and you fail to meet margin requirements or repay a margin loan. In such cases, the broker may have the right to sell your securities to cover the shortfall.

Additionally, if you hold securities in a cash account and you fail to settle a trade or meet other obligations, the broker may have the authority to sell your securities to cover those obligations.

The only way to be secure is to DRS your shares. I still hold some in my brokerage account though, just in case.