It has nothing to do with free speech or rational expression of oneself

Being rational is not a requirement for free speech. There are clear rules about what is considered free speech. Burning books might be tasteless, but it should not be illegal, regardless of which book it is.

Why can´t people just show at least a little respect for other cultures?!

Respect needs to be mutual. These people who are offended by some burned books will happily disrespect other cultures or beliefs.