Re: So why the controversy
Mostly because linking into GPL is something programmers choose to do. There are two important elements to this which I will take separately:
1. Programmers, not users. The SSPL comes into effect when you run the software on a computer if you use it for a certain purpose. The GPL does not care when you run it or why you did it. In fact, you are perfectly free to include the GPL software in your software as long as you don't distribute it, I.E. to use internally. You don't need to educate anyone putting the software to use on what the license means. You only need programmers that might modify or use it in their own software what it means. They have probably seen open source before, so they already understand what restrictions apply to them.
2. Choose to do, rather than find that they've done: If you choose a GPLed dependency, you know you did that. When you pick something off of GitHub, you know that you'll have to read the license because it can be something proprietary that you are not allowed to use, so you know when the terms apply. You can understand the conditions on what this applies to, because it's anything you're linking this with, so you know what you have to put under GPL if you go ahead. With SSPL, neither applies. You may not know whether you are in the set of users that have to put software under a certain source, especially because all you did was install it on a server. If you decide that you are one of the group that has to do that, you don't know what comprises all the software the SSPL is demanding, and it's mostly going to be unrelated stuff written by other people (which you couldn't put under the SSPL anyway). Unlike the programmer and their own code base, it's the user trying to list all the pieces of software that come under a nonspecific category, which the average nontechnical person, even a Linux user, has no hope of doing. Even the most familiar person will have to spend a long time sorting things in and out of the list.
Many of us who care put some importance on the Open Source Definition. The GPL meets this definition. The SSPL specifically violates this part of it:
9. License Must Not Restrict Other Software
The license must not place restrictions on other software that is distributed along with the licensed software. For example, the license must not insist that all other programs distributed on the same medium must be open source software.
It also violates, both in letter and in spirit, this part:
6. No Discrimination Against Fields of Endeavor
The license must not restrict anyone from making use of the program in a specific field of endeavor. For example, it may not restrict the program from being used in a business, or from being used for genetic research.
This is what I care about and the reason why the SSPL is not open source.