Why are researchers at some think tanks titled professor?
The word "professor" is most often used by universities, but the content of the word more generally refers to a person whose job is some mixture of higher-level education and research.

Thus, while it is unusual for a place like the Santa Fe Institute to call some of their employees professors, I would judge it to be a reasonable description of their work given that the institute is well-respected in the academic world as both an educational and research institution.  

Most other industry or "think tank" organizations do not have a similar mission and it would thus be less appropriate for them to give such a title to their staff.