I've been keeping out of this discussion, and not just in this forum, because it's one of those things where...you either get it, or you don't. Frankly if you think it's okay to burn holy books then, as long as you're consistent (surely you wouldn't be pissed off if mosques start burning bibles right..?) then there's no helping that. In fact, I feel the same way...I think symbols and the destruction of symbols have cost a lot of lives over the years for no real reasons.
That said, plenty of people, in fact the majority of people, do believe in and understand the power of symbols. Burning books is especially egregious, as it has long since represented the destruction or the condemnation of the knowledge contained within those books. Essentially, burning qurans is equivalent to saying "islam shouldn't exist", and hopefully people can see why that would ruffle some feathers.
The most relevant argument here, though, is not that this is illegal (although its legality is questionable) but that it's immoral. Civilized societies should tolerate all peaceful religions. Civilized societies should not view book burning as anything less than barbaric. That's all. The right of free speech doesn't mean all exercises of that right are ethically equal; Bill O'Reilly has the right to insult black people as he regularly does, but that doesn't make it okay by any stretch of the imagination. The same concept applies here.