No, it's not the same as saying someone can be ignorant of the law. It's saying that there needs to be reasonable warning that a law would be broken. This isn't a case of "I didn't know there was a copyright law." It's "I know there is copyright law but had no indication that I was actually breaking that law." As Martin said, it's shifting the burden to the copyright owner to warn people that they would be breaking the law if they use the image without permission, rather than putting the burden on the person to check if there are any restrictions on the usage of the image (which may be easy in many cases, but not so clear in other cases.)
I'm sorry Lenny, but I'm going to respectfully disagree with you. Copyright is created simultaneously with the work; irrespective of the nature of the work, the copyright comes into being as soon as the work does. That is the law. Extrapolating that point to a logical conclusion, saying that "there was no indication of copyright" is a valid excuse is indeed allowing ignorance of the law to be used as justification for breaking the law, since there doesn't need to be an indication of copyright; the fact that the work exists means that copyright exists. The holder of the copyright can choose to do a wide range of things with it, including license it under the 'creative commons' umbrella, or other 'free use' doctrine, but in this case the creator did not do that. Placing the burden on the creator to protect his work with signs is no different than saying to someone whose house has just been burgled, "Well... you didn't manage to keep the thief out, and you didn't have any "No burglary" signs, so... it's your fault.