Musk himself has recently used a similar example, asking journalists how they'd feel if somebody actually got hurt as a result of doxxing on Twitter. So how would folks at Twitter feel if they ignored an FBI report of activity that then led to a terrorist attack? They wouldn't just feel bad, they might actually be liable for helping to facilitate it. Companies do all sorts of things to avoid potential liability, or forego doing things even if those things are perfectly legal and the company would prefer to do them otherwise. It's not weird or nefarious at all for a company to err on the side of caution when the receipt of information increases their potential liability.
It's also extremely hypocritical of Musk (or his fans) to oscillate between maximalist free speech and protection of privacy, invoking extreme examples in both cases, clearly according only to which one suits him personally at any particular moment.