They absolutely can if they operate like banks with a fractional reserve. All of them do, they cannot resist the temptation to "efficiently allocate" the huge amount of money they are sitting on.
It's totally illegal and the SEC is chatging everyone they can reach. But most are out of reach, because they are outsdt US and the Presidents refuse to use the military to defend USAns against finacial warfare.
The closest real world analogy is probably https://en.wikipedia.org/wiki/MF_Global : same story of illegally transferred client funds from what should have been a plain custodial brokerage.
