I guess it's just being a bit cautious when posting pictures in an environment that can be seen by potentially such a huge audience. None of us 'normal' folk are fully up to speed with what the various sorts of identity scams are out there.
I don't bother blanking my plates out, but I understand those that do....
In AZ you used to be able to give the DMV a plate number, and for a couple of bucks they would give you the name and address of the owner. That changed when unfortunately someone finally used that information to track down a lady he had seen and killed her.