How is Oanda able to offer 3% APY on unused margin?

I saw the advert as I was logging into my live account.

Earn up to 3% APY* when your average unused margin is over $10,000. See terms for details. Now your money earns interest even when it’s not invested.

Beats any bank account I have by a mile. I mean, I don’t have $10K around to add to my trading account, but I wonder if people use this just like a high-yield savings account. Works the same way right? I thought only banks do this sort of thing. Is this common with other brokers?