PayPal has been doing this for more than a decade now. I wouldn't be surprised if its part of their business model to hold funds indefinitely and to collect interest on them.
Besides, if that was their business it'd be in their S-1 and it isn't. (IIRC it says they keep customer funds in interest free accounts)
exercise for the one's imagination - you come to a banker and say "i'm thinking about keeping with you an N billions of dollars, interest free. Any ideas?" (in mid-199x i did some work around large Russian municipal funds to be kept in banks, and what i saw helped develop my imagination :)