Exactly. If you live in a city of any size, how many thousands of people a day can see your license plate number. Blacking out the license plate gives people a false sense of security.
...on the net? When you see somebody post pictures of their car on a forum or something they usually take the time to edit out the license plate number. Why bother? If you can see their plate when they drive down the road why does it matter if you can see their plate on a picture on the net?
I doubt most people on the road have photographic memories. Basically, if the car is pictured on the internet people can take their time and glean a lot of useful information from the picture. With the plate number and description of the car, people could file false police reports or go to the DMV for that state and ask for the registration address of that vehicle. That in turn could allow for someone to stake out the car to steal it, or someone could just use the owners name and address to try and commit identity theft.
Because that license plate number is as valuable as someones social security number. Once that number is obtained, anyone anywhere can do anything with it.