I agree that the philosophical points raised here make a lot of sense, but I also tend to think there is a fundamental flaw in their application to those involved in the acts discussed. Specifically that the mindset and agenda of those involved is in any way the same as that of those in HG.
While there may be one or two highly principled individuals who see these items as a legitimate target and a means to gain some sort of publicity/notoriety, anyone I've had the misfortune to meet who has vandalised public items has been drunk, dared or just plain bad, and happy to spoil others enjoyment to get a kick.
The other group comprises the thieves, and these seem to fall into two caregories: one lot that steal to order or in the the hope they'll find a buyer in one of their past clients, and the others that steal items for themselves. These can be surprisingly ordinary folk, as I've found when passing back gardens I shouldn't have really have been that near to, when doing a bit of UrbEx.
Anyway, my real point is: don't think like yourself when trying to work out the reason/motivation, it won't work. Get into your subject's mind, and try and think like them.