As mentioned in other comments by Spaniards weighing in, it's not really a big problem as in that's happening all the time. It is a multi-faceted issue, and for some, a philosophical standpoint stemming from the economic crisis, gentrification and speculation, and related problems.

Take in mind that the post shared here is written by a company that provides listing services.

Housing is more of a right in Spain than in other countries, and at least in Catalonia, there is precedent that it supersedes property rights.