Gamedev is really a special case (if we consider Web Development to be the "common case"). The fact is, when a company open sources one of it's projects, it expects to "benefit" from open source, i.e. taking community contributions, and make these contributions into its other closed-source products to generate profits. GPL forbids this, and naturally most companies open sources with MIT/BSD.
And follows a repository on GitHub, many issues and PRs, CI, periodic releases (feel free to replace "GitHub" with "GitLab" or some server on some university, replace issues with mailing lists etc.) - that's what we typically expect from an "open source project" backed by other companies, in some other industries (most notably web development).

It doesn't work this way in games (at least for now), EA is only releasing part of the code for modding purpose, EA doesn't expect an "open source community" established on this codebase, and I also don't think EA has the intention to take contributions directly from community (by "directly" I mean, in code). If you have something you don't like about the game, post a comment in Steam and if you are lucky the devs are going to fix it themselves. It has always been working in this way in games, and it's not changing despite the "open source" action.
So in this case, GPL is actually more helpful for EA, as illustrated by other replies.

But what I really want to say is, being a C&C modder myself (I've been modding RA2/YR from 2009 and also briefly worked on C&C3 later), I wholeheartedly support EA's decision to use GPL. Not because it's good for EA, not because I'm also a fan of RMS, it's because of the specific situation of the modding community.

If we take a simplistic view of a mod, it's roughly composed of "assets" and "code" (the same could be said for full games but it'll be more complicated). For assets, there are awesome free content creation software such as Blender and Krita, and tons of learning resources on the Internet. One can easily become a quasi-professional artist given enough exercise (I, personally, have undergone such a process). Well there is also things like music, but they all work the same way in which you "just create something in some external application (which is usually very powerful and extendable) and throw it in the game". You are not really limited in terms of asset.

The real problem is in "code" - modders are ultimately constrained by a proprietary engine, which heavily limits the possibility of mods. Just think about it: when you're making a real game, you can think about "What is the most interesting stuff I can come up with?" and then implemented it in the game engine. But when making a mod, whenever a fancy idea pops up, you're obligated to consider "Can it be implemented in the engine?". Unfortunately, most of the time, the answer is no. And this goes on and on, till a point where your imagination is imprisoned by the capability of the engine, that you lost the inspiration to design anything novel.

Why the engine is so limited? I think we can just imagine the scenario: in the crunch of getting the game finished, the devs don't really care about moddability, they just want to get it working ASAP, then they do all kinds of hack all the way along. In the end the game works, but modding it sucks. They hardcoded many stuff directly into the game engine binary, which is perfectly reflected by this set of search result: https://www.google.com/search?q=hardcode+site%3Amodenc.reneg... ... Oh and this is only about modifying existing stuff, we haven't started talking about adding a completely new system yet ...

To overcome these limitations, modders did a tremendous amount of efforts over the years:

There are attempts to reverse engineer the binary and patch it for workarounds: https://modenc.renegadeprojects.com/RockPatch

Then this approach evolved, they analyzed the binary to get the class hierarchy of the game, then use C++ to write new logic, inject the compiled routines into the binary: https://ares.strategy-x.com

There are attempts to rebuild the complete source code of the engine by incremental reverse-engineering: https://github.com/TheAssemblyArmada/Thyme

There are attempts to rewrite the whole game: https://github.com/OpenRA/OpenRA https://github.com/OpenSAGE/OpenSAGE