Basically, it boils down to: "Why Oggh try get burning wood? Fire bad! Everyone knows!"
Taking apart a toy to see how it works is something I daresay all of us have done at one point or another.
As far as I'm concerned they're in the clear. Have you never felt the joy of tinkering with something? It's awesome when you can sink your teeth into a problem and after a long, arduous fight finally find its solution - all the while giving new functions to a product you paid for in full.
They're true tech enthusiasts for the most part, and in my opinion they're sorely needed in our times, with DRM and IP shenaningans encroaching us from all sides.
They're free to do whatever the fuck they want to with the hardware they obtained legally. I mean, look at how insanely useful the XBox 1 became as a media center once Bunnie and others gave coders access to its innards.
Or look at
this. It serves no purpose at all. It just plays Guitar Hero in a thoroughly unorthodox way. Should we bar him from inventing something like it just because it defeats the purpose of the game system?
Reverse engineering, just like all research, isn't morally ambiguous per se. It's the application of the knowledge, mostly by uninformed others, that really gives the knowledge meaning. The homebrew guys are harmless. The pirates that abuse their findings, however, are not. It only takes one guy that tinkers with a system to break its security and enable piracy. Put it into perspective: Our cozy little mudball houses vastly in excess of six billion individuals. Even if only a ridiculously tiny fraction of those have both the means and the motivation that still amounts to a ridiculous amount of possible "leaks". There will always be one guy that spreads the knowledge. Always. So let's at least reap the benefits with informed and educated tech professionals. They need to learn their stuff somewhere, so why not let them tinker with whatever they find in their homes.
Humans will always try to expand and explore. It's in our blood. Without this instinct we'd still be flinging poo at each other in some ditch. No curiosity, no progress - it's that easy, really.