The discussion is around Parked Domains i.e. a user buys a domain and has not added any content to it yet. Still he/she might get a few type in traffic and they monoetize this traffic by showing Google Ads on it. Google considers such sites to be part of the Search Network and not the Content Network(Atleast that is was true 3-4 months ago and I pretty sure it is still true). So putting them on the list of excluded sites as that list work only for the Content Network.