This type of initiative should never be open source.
The issue is, it comes with costs, to things like SEO.
It's not hard to lock a computer system down so only logged in users can see content. It's a lot harder to lock them down so that only logged in users AND a select few entirely unauthenticated scraping tools can see your site to assist you in promoting it while OTHERWISE people aren't able to access it.
It makes sense economically since the part of the population using such services is so small and may have specific demographics that justify specific treatment just from a marketing POV.
Then you want search engines to get the whole content (okay at least before they started to summarise everything but the kitchen sink) - that will always leave a loophole.
When previously nobody would have gone through the effort to build and maintain a custom scraper against a moving target, for some small to medium size city's newspaper, now it's just one of a myriad of 'scraper profiles' you can have a near fully automated tool create.
The CMS usually allows search engines to get all content. The feature is to know human browser operators and make them pay, or know archiving/unblocking services and block them.