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.
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:
http://daringfireball.net/linked/2010/04/19/gizmodo-rumors
So quite literally since he found out Gizmodo had the device he has been offended by it.
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.