If its not public, you have minimal chance of discovering it if it isn't provided, so while it is illegal for them to withhold it, you'll never know it existed and was withheld unless someone who does know of it -- almost certainly the police themselves, or the person shown in it (quite possibly, the real criminal, who would be revealing their own identity by coming forward) -- actively reveals it. So, illegal or not, it makes it much easier for them to get away with concealing exculpatory evidence.