By default, a connected USB device capable of mass storage will be treated as a storage device (with associated
/dev entry, etc). Cameras (and e-readers, and assorted other devices) treat being connected using the mass storage protocol as an "unusable" state, since you could be writing to device storage from the computer at the same time as the drive is in use by the device. Some other operating systems don't connect to storage devices by default in this way, so the interface on the device is often misleading about what's happening.
To work around this, you can
eject the storage device. When you do, it will stop being connected as a mass storage device, but retain its power supply. Something like:
sudo eject /dev/sdb
(for the appropriate device name) will do what you want. On a Mac, the drag-to-trash or right-click-eject options should do the same.
If the device supports other protocols as well (HID, video), those aren't affected by the ejection. Only the storage device disappears.