Uhh yes it is. I have iCloud right in front of me. Both the desktop and iPhone ones have "Photo backup" and "Camera roll" respectively turned on initially.

You have to go to iCloud -> Storage and Backup -> Manage Storage -> Camera Roll to turn it off.

Unless you mean that iCloud backup itself is not turned on by default, which is accurate. But the way you worded it it reads like if you have iCloud backup, it won't backup photos until you expressly tell it to which is inaccurate.