I feel that when you ask an employee for permission, they automatically assume that you are up to something negative. They are usually not in a position of authority to grant you access to anything anyway. Now that you asked for their permission, they feel like they are personally responsible for whatever you do with the photos. The easiest way for them to CYA is for them to say "no."
For things like restaurant menu boards I wouldn't be worried. Installing apps on computers I do not own (and owned by a very large corporation) without explicit permission? Not so much
I'd even say the most clear definition, Downloading an app from an external website and executing it, is pretty clearly in the allowed list - These machines exist to demonstrate Apple computers, and Installing an app is a not entirely unheard of action after purchasing. There's a reasonable expectation that Apple could prevent it if they wanted, and that the 'harm' done to them is virtually zero - They re-image their machines all the time. So where is the boundary?
Obviously, in this case, it's where people started feeling creeped out by it... but this was by no means a secret service investigation. This was a high-school level prank, and it saddens me that we live in a world where people would take enough offense - where people are so mistrustful of one another - where people are so litigious, that federal law enforcement is called in after such.
However the fact that he felt he needed to hide what he was doing by using tumblr etc makes me feel that he knew there was a line (however blurry it might be) and that he was crossing it.
"Sometimes I would open another tab and load Flickr or Open Processing so I had an excuse if someone asked why I was comparing every single computer."
If he truly thought what he was doing was legal (or morally right), he wouldn't have had to do this. Instead, he chose to hide the fact he was installing software on Apple's computers.