To be sure, WaPo also sells data to their advertisers. At the same time, WaPo has broader coverage and probably needs a larger newsroom as a result. Ars Technica is a subsidiary of Conde Nast, and Conde Nast as a whole has paywalled publications (like New Yorker).
In general, I think it's perfectly fine to promote paywalled content on HN so long as the content itself is good. We have to fund quality reporting/writing, and HN can serve as a curation mechanism to surface "content worth paying for".