But it's just religious books. You can burn Darwin's "The Origin of Species" or Kant's "Was ist Aufklärung". But you aren't allowed to burn a bible or the koran?

That's just stupid.

1. It is bad for the environment. Too much co2 production in the process. If you must destroy books, recycle them!

The CO2 produced by this is extremely marginal. Some single occasions of this won't have significant impact. Despite that: books tend to rot after a while, thereby releasing the stored CO2 anyway.

2. The Nazis did this very popularly. I always get reminded of that, when people burn books.

The Christians also burned books on multiple occasions. As did the communist revolution under Mao Zedong and a bunch of other lunatics throughout history. If we should agree that burning books (as a form of protest) is a bad thing, then include all books and not just some religious ones.

I agree with your third point. However, it's a very visual and "spectacular" (meaning it draws attention) way of protest.

4. I think most countries ban burning houses, even if it is infringing the freedom of speech. Why should it be different with books?

Burning houses does significantly more damage and poses high risks of further collateral damage than burning a book. Moreover, houses usually don't carry and spread ideologic views.

Houses are not a medium to spread information. A book is , it means something so it is speech. Just like burning the US flag is allow because the first amendment allow us to judge and say fuck to our government.