All copyleft licenses have the same principle; to promote openness. This is explicitly not the same as "use it however they want".

I think the motivation to not have your open source code used in a closed source product that is sold for profit should be obvious.

Also, the motivation seems to be a desire for building a bigger community which gives back its improvements. ZeroMQ for example sticks to LGPL for the very reason - they believe that it is easier to build a community and discourage "dark forks" when software is licensed under GPL-like license. (See http://zeromq.org/area:faq#toc4).

They have tried hard for the last year it relicense zeromq under a much more liberals license, but have had trouble getting all permissions.

Additionally, the requirement for being able to sell a business friendly license, seems to imply that any community code contributions must now assign copyright of the contribution to the project owner (CLA?). Otherwise, wouldn't you then have features locked to the EUPL version? (presumably...I am not familiar with the EUPL at all though)

I want you to benefit from my work only if I also benefit from your work. If you seek an unfair advantage over me, I will not cooperate.

Under a copyleft license you CAN use it however you want. You just are obliged to open source any modification you make to it.

I'm not sure how the "derived works" clause applies to node.js projects using this project.

You're obligated to also distribute sources to changes if you distribute.

You can implement single-image clustering on top of the Linux kernel all you want, and keep your changes - so long as you don't distribute the software. If you do, you must distribute under the gpl.

GPL and LGPL require that users of your software have to be able to relink with their own version of the copyleft code. It's not enough just to provide the source (for compiled languages).

The open source version acts as advertising for the paid version. It also allows programmers to imagine that they will get paid (without even having to do "business stuff" like marketing) and get open source cred at the same time. This rarely works but the other open source business models never work, so it's the leader.

Others have pointed some reasons, but there's also another important one.

Copyleft is a promise to he end-users that - no matter the circumstances - they won't be stuck with whatever they were served, but would be able to maintain or hire someone to maintain. It's not only freedom to adapt and modify the code, but also - when a project is a library - a freedom to not be stuck with ancient bug-ridden insecure builds if there are newer versions.

So if you ask "why copyleft?" think how much abandonware with statically linked libraries (e.g. ancient OpenSSL) you've seen.