I do understand the need to scan the laptop separately to its bag, but this honesty policy just failed hard. It's a stupid system. The only workaround is to keep everything in sight until you're 100% ready to step through the scanner, and then watch the other side of the belt for all your stuff.
The fact that the TSA handed the laptop to someone other than its rightful owner lands them straight in the blame for this. At the risk of increasing the time required to clear security, a big fuss was made last year that all electronics must have charged batteries so that it can be demonstrated they work (not that this solves the key problem). If the TSA want to personally pick up someone's laptop, they should ask the owner to power it on and enter the password to prove it's theirs before they hand it back. Yes, this also incurs a privacy issue (intentionally defeating your own security system), but as repeatedly demonstrated, border controls have some quite far-reaching powers and can force this anyway.
Edit: also, shame on the security people and the police for dismissing the idea of a crime being committed. Intentional or not, this is theft. As others have commented, it's possible the two women were working together to steal a laptop, but even if it was an honest mistake, it should be investigated as if it were intentional. And the reluctance of the people involved to give case numbers, log it etc. just reeks of 'we lost your bag, thank you for flying Delta Airlines'. I sincerely hope this guy gets at least a check for a new laptop and an apology.