Well, in Google's defense, they DO sync to your camera roll and Gallery just like iOS devices do. The problem is, you want to use iCloud instead of Google+ or Dropbox.
To do what you want, unfortunately, would require iCloud to come up with a stand-alone version for Android, and we all know that's not gonna happen.
You could try 3rd party apps that download your Android pictures to your PC when on the same WiFi (or USB) and then transfer those to your iDevices when those connect to the PC, but it's the same issue I'm assuming you have with using services like Google+ or Dropbox.
Now, obviously I'm no iOS expert, but the only way to do what you want using Dropbox, for instance, would be that iOS allows other apps to show content in your Camera Roll, just like Android does with their Gallery. So it really is an iOS limitation. PS: I just looked at my iPhone and yup, there's no option to show external content in the Camera Roll like there is on Android (there you can see your Google+ or Dropbox photos in the Gallery).
If what you want are your photos to be always downloaded to your iPad, Dropbox has that option, to 'sync' a certain folder to your device. That way you can download your pictures to your iPad automatically and they should, in theory, show up in your Camera Roll since they're physically in your device.