1) Publishing and distribution costs are far less online than with print.
2) News organizations have a long history of generating revenue by advertisement, and this is especially the case for many highly trafficked websites.
3) The owners of the newspaper benefit from a wider reach, because it allows them to better influence the public.
4) News websites, in many cases, either used to be free or still are.
5) The price of a full subscription may not be fair for someone who only wants to be able to read the occasional article linked from HN/reddit/friends.