Imagine this scenario. You want to write your own code ProductX and link it with some GPL code. This brings ProductX also under the GPL license.
You can put ProductX on a server and never actually distribute ProductX to anyone. By not distributing ProductX, you don't have to distribute the ProductX source code to anyone -- although technically it IS under the GPL. Since you're not distributing the binary, you don't have to distribute the source. Yet the public can interact with your server and make use of ProductX's services.
The AGPL prevents that. If you write your code ProductX and link it with AGPL code, then ProductX comes under the AGPL license. But now, merely letting the public interact with ProductX on a server requires you to distribute the AGPL source code to ProductX. Now anyone else has the ProductX source code and can compete with you.
The AGPL is also a way for the author of an AGPL library to make money. A library such as iText. If you want to use iText with a proprietary ProductX, then you need to buy a separate commercial license for iText.
If the developer's intent in ProductX is to keep the source code private, then the developer cannot link ProductX with any AGPL licensed code such as iText -- unless the developer is willing to pay for a commercial license to iText.
A developer should never have code that they didn't write themselves, such as a library, unless that library is under a license that the developer can always remain in full compliance with. That includes proprietary and commercial libraries as well as open source libraries. While AGPL is "open source" if you cannot comply with the terms, then don't use it.
Finally, avoid any code you find on the internet that has "no license". If you use such code, the copyright owner of that code could sue you for copyright infringement. "But wait!" they say. "I would never sue anybody, I just want as many people as possible to use my code, so I don't put it under any license." I say: If you're not going to sue me, then put that promise in writing. it's called a license. If you're such a good guy and aren't going to sue me, then put it in writing like all other open source licensed code.
Thanks for the explanation! Does anyone actually release code due to this license (which they wouldn't have released otherwise)? I'd imagine generally if a commercial developer saw a license like that they would just choose not to use the AGPL code. I know I won't use any code or libraries which place these sorts of obligations on me as a developer, unless it's in a product that I was planning to release the source of anyway.
If a commercial developer saw a license like that... they could pay for a commercial licence. Presumably a commercial developer thinks software is worth paying for.
> if a commercial developer saw a license like that they would just choose not to use the AGPL code.
Except for the server bit, the restriction being described is called copyleft. And it appears in all GPL licenses, except LGPL. Some of the major OSS in use today is released under it: Linux, MySQL and MariaDB, VLC, and of course all the FSF software, such as GCC.
The AGPL is an interesting idea in principle, but it is an ecosystem-level coordination problem if you apply it to libraries. Application that include those libraries also need to be AGPL which means any existing application that wants to use it must be re-licensed to AGPL and only use libraries that are AGPL-compatible. Nobody wants to be a first mover in that direction.
Plus it is untested in court just how far its provisions go.
