Obviously Fidelity is taking on some risk by letting you do this if the transaction does not clear. However, normally, customers depositing are going to buy a variety of normal, not too volatile assets. If your deposit doesn't clear, they just sell whatever you bought and get their money back.

If a large percentage of their customers were new, young, fairly low income (and thus more likely to have their transactions bounce), and they all wanted to buy long positions in a stock that very likely could drop 90% in the next two days, the risk is far higher.