Burning books, in general, is bad. However, as long as the burner *owns* the books, having lawfully purchased or received them prior to the event, it isn't and shouldn't be unlawful - just the subject of scorn and derision.
However, since the printing press was invented, the notion of burning any one copy of a book whose copies in print run in the millions...is notably less obnoxious. If the said book is posted on the internet, it is created and destroyed hundreds of times daily as hard drives across the land are re-written, de-fragged, and re-formatted. Burning a copy, or even a couple of hundred copies, is meaningless in any logical world barring sentimental value or some unique characteristic of a specific volume.
It is in other words, magical thinking, an endeavor to be firmly discouraged (particularly and especially when participants live in a world of fantasy and get violent when the folks around them fail to go along with their particular mythology). Magical thinking enforced by violence has never worked out terribly well, historically. Remember the Crusades.
Further, if a bunch of religious sorts whose skin is so thin and faith so fragile that the mere burning of a non-unique volume sends them into a literally violent hissy fit...then, in the same manner we must deal with a rabid dog or a zombie invasion, I suggest the appropriate response to a public hazard of this sort is to fix bayonets and hunt over bait until one runs out of crazies.
That the more enthusiastic followers of this particular version of woo-woo thinking killed off 2,974 Americans (roughly as many as were killed at Pearl Harbor, and disproportionately civilians) nine years ago today - does not inspire me to any greater sympathy for their tender delicate feelings in the matter.
I find book burning reprehensible, but in this one instance, particularly in light of the threats and bullying, am a lot less offended than usual (though I find the individual at the center of all this rather offensive, and in my opinion based on his media appearances and actions, a loon). Emulating Neville Chamberlain and his policy of appeasement does not seem a good idea - at what point does one stop paying the danegeld and become simply owned by the dane?