If you think that's griefing, you haven't seen the wand of equal trade from TC3. Nice house made of stone ya got there, let's replace that all with tnt.
But yea, I can see that as a problem, especially when combined with the wand mentioned above, as they could simply replace a stone structure with wood and then set it on fire.
I think the real issue here though is why is it melting obsidian into lava?