He has a very strong personal attachment to Apple and it looks like he thinks Apple was wronged. I don't really understand cheerleading a $230b company to prosecute a blogger for photographing and returning a phone lost in public, but he's not alone in that. People take Apple personally.
The humorous thing is Gruber leaks information about Apple all the time, so it must only be wrong in his mind when you have proof (or well, show the proof). Apple got their hardware back and in the end all that happened was rumors were confirmed. It's an ego hit for Jobs, but he could use a few of those.
Gruber is not offended about the phone leak. Gizmodo's analysis of the phone hardware they obtained is legitimate, even though their methods in obtaining said hardware are shady-to-illegal.
What Gruber and a lot of other folks are upset about was the entirely unnecessary outing of the specific engineer who lost the prototype phone.
That's not true, he was offended well before they outed the engineer. He actually never even mentioned the outing in the OP.
Gruber retweeted this at 1:41 AM on the 19th, 18 hours before the outing: "Here's an interesting fact: in California, the finder of a lost item is required to tell the police and turn it over to rightful owner."
He called it stolen on his first post about it, just after Gizmodo posted:
Again, though, the method is what he says offends him.
And, honestly, in such a case as this almost anyone is going to have the first reaction along the lines of "this device is almost certainly stolen" no matter how good the purported finder's tale may be...and this one's excuse seems pretty threadbare.
Engadget (apparently) reacted by publishing the photos that accompanied the shady proposal; e.g. they got the story but stayed out of the morass. Gizmodo (again, apparently) reacted by accepting the shady proposal...this is what dismays Gruber who, as someone noted above, is going to be painted with the same (admittedly) broad brush as a result and suffer its consequences, large or small.
The humorous thing is Gruber leaks information about Apple all the time, so it must only be wrong in his mind when you have proof (or well, show the proof). Apple got their hardware back and in the end all that happened was rumors were confirmed. It's an ego hit for Jobs, but he could use a few of those.