Pirate servers are disliked for three reasons:

1. the players of the official version represent Blizzard at the level of gods, and anything else is considered heresy.
2. The availability of open source, allows it to be used by unscrupulous organizers.
3. Highly skilled developers of emulator, unfortunately, very poorly know the game and introduce unacceptable mechanisms and algorithms, disrupting the gameplay.

Do you want to fight it with video?

A video review from Valsery has been published on YouTube about this article:

https://www.youtube.com/watch?v=U0Nfn8M5jZc

It has English captions, however it would be nice to get translations in different languages.

If you want to help, please:

1. Go to this page: https://github.com/FrancescoBorzi/why-I-hate-wow-private-servers/tree/master/video-captions

2. Add File

add or upload a copy of english.txt renamed to your language with all the texts replaced with the correct translations. Do NOT touch the timing of course.

GPL doesn't say changes have to be released to the public. What it does say is you have to distribute full source code anytime you distribute. As private servers do not distribute the software in any form, it isn't applicable. Even says this in your link:

In purely private (or internal) use—with no sales and no distribution—the software code may be modified and parts reused without requiring the source code to be released. For sales or distribution, the entire source code needs to be made available to end users, including any code changes and additions—in that case, copyleft is applied to ensure that end users retain the freedoms defined above.

You say

In simple words, this license says the following: use the code to do whatever you want, without paying anything, as long as any modification to the original code is also released under the same license. Basically, the license of these projects requires those who use them to publish any changes to the public.

but GNU GPL does not require users to publish changes. It is only required if they release their emulator in binary form which private server operators don’t.

There is also GNU AGPL which requires to publish source code if it is used on publicly available server, but it isn’t MaNGOS license.