It's not EA making the call. It's the fact that the Frostbite engine incorporates some third-party tools (basically: tools made by someone outside of the Frostbite team).
As a (very, very) rough example, assume you're working on a game engine and want it to support feature X, Y, and Z.
You have two options now. You can build the engine from the very bottom, using your own time and team to build every single detail, even if other games have solved the same problems before. That's expensive and time-consuming, and will ultimately inflate the prices of games you make, because now you have to pay the salaries of everyone who worked on making those parts from scratch. That's closer to what Bioware did before using Frostbite, and it's part of the reason why Origins took something like 6 years to make.
Generally the more cost- and time-effective approach is to buy existing tools that have already solved these problems. Again, this is a super simplification, but imagine our fake game engine decides to use a third-party tool to handle some aspect of combat.
The studio that makes that third-party tool makes a living out of selling their tool to combat designers. The last thing they want is for someone to get access to their tool for free. So they sell their tool to people making game engines, but they say "please don't expose our tool to your players, because it will destroy our business model".
Because game prices haven't kept up with inflation and games need ever-increasing technical specs to keep up with the market, you agree to their terms. The third-party studio is happy because they have economies of scale, so they can afford to sell their tool for less than it would take the engine studio to make it from scratch. Your engine studio is happy because you can ship on time and on budget, which means you can pay your employees.
But, when that kind of transaction happens, it does make it harder to support modding. Now you've got to keep this separate, completely-outside-of-your-control third-party studio happy. And usually "happy" means "not letting players use their tools".
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Disclaimer: I don't work in video games specifically, but this is how third-party tools work in other areas of the software industry.