This is the license the X-Pack code is under: https://www.elastic.co/eula
Nor would I put it past them to consider legal demand letters as part of their sales funnel.
Suing users is a pretty terrible business practice and not, at all, a funnel creation opportunity.
Re. License, there are a few FAQs on https://www.elastic.co/products/x-pack/open#faq that may help clear up any misunderstandings. Free features are free and governed by the Elastic EULA but are enabled by default. Paid features are available through an opt-in trial and, if they help solve a problem, can be purchased.
<disclaimer: I work at Elastic in Developer Relations>
They have an incredible product that I can't live without, it's just incredibly painful trying to actually pay them money for something.
Honestly, I think their real play here is getting community contributions to fix stuff that's very broken and also to edge out Siren Solutions. SearchGuard, Kibi and Sentinl are all _very_ good tools.
If you want, I'd be happy to talk more about it so that we can understand where we can improve. Feel free to email me tyler.hannan<at>elastic.co
<disclaimer: I work at Elastic in Developer Relations>
There actually still is a pricing page somewhere on the site, but you have to dig for it, and I don't think it's correct anymore.
What drive the community to involve in their software
For writing clones or alternatives, it might be better not to read any proprietary code: https://en.wikipedia.org/wiki/Clean_room_design
Check many solutions can give an idea how to solve the puzzle. Make this projects open source can give me an idea to create plugins. Ofcourse still need think for yourself if it fits your needs, quality and if you not are struggle with possible copyrights.