Because PoS, which is organized by the International School for Advanced Studies based in Trieste, Italy (SISSA), is an Open Access proceedings collection, INSPIRE is free to distribute its content to all users. You can find out more about PoS from its website: http://pos.sissa.it/POSwhat.html.
The field of HEP is making some big moves in the direction of Open Access this year. Keep an eye out for even more fulltext content on INSPIRE in the near future.
Are you still learning how to search in INSPIRE? Here are three ways to take a look at the PoS records we have:
1. In Journals, search for Proceedings of Science or pos.
Click on the journal title, and then click the link for "Articles in HEP".
2. Using SPIRES-style searching type find j pos in the simple search (default search) box.
3. Using Invenio-style searching type journal:pos in the simple search box.
More tips for searching in specific journals can be found here: https://inspirehep.net/help/search-tips#journals