USB power behavior in Mavericks is different?

I just noticed that now that my '13 MBA is running Mavericks, I can still charge my phone using it's USB port even when the notebook is unplugged and in sleep mode. I think it's actually a pretty desirable behavior, as I can use the notebook as a giant battery for my phone or any other device.

Did anyone else notice this? Is this even a feature or a bug that happens to be useful to me?