Police are legally allowed to possess contraband in the course of an investigation; private-sector entities aren't
I consider that to be a real problem with the current laws. There are some data, which can be used for good or for bad purposes. Google has lots of data, they also have the computing architecture to store and process all of this data, and they have the expertise. I firmly believe Google is in a better position to do this sort of data mining than authorities. It is not just about images of child abuse, but also about correlating that with other data, which would not be illegal on its own.
If Google by performing mining across a little bit of child pornography along with lots of legal data is able to produce an output, which can track down the people who were abusing the child in the first place, then I consider that to be a good use of the data, regardless of what the law says about that practice.
The potential for Google to help track down some of these kids and get them out of the abuse is so important, that it is unfortunate that such efforts are jeapordized by the current laws. That makes it only so much nicer to hear that Google is doing an effort in this area. Whether Google is breaking the law or not in the current effort is not important to me, as the goal of the effort is to go after much worse crimes.
I consider abuse of children to be a much worse crime than possession of child pornography. Most people agree that it is worse, but some people seem to think it is not that much worse. Would the average person think it was ok to let 100 people guilty of possession of child pornography go without punishment, if it meant one more person guilty of child abuse could get caught?