Apple won't do the police's job investigating a crime. That would be a huge liability problem. They are more than happy to assist the police, and apparently the Airlines and other lost and found agencies. Nice to know.
It may seem resonable to you, or me, that Apple should tell us where/if they've seen our iPads. After all they have a record of who bought it. But they don't have a record of whether it was sold it, given away, nabbed by a disgruntled ex-spouce, repossesd for a debt, etc. If they gave away information about the iPad without a warrent they could easily find themselves on the pointy end of a legal department. So they insist, not so unreasonabaly, that you file the claim with the police and have them ask.
Good hearted people and companies that volutarily report missing items to Apple are another matter. It's good to know that they exist.