> there's plenty of other training data in the world.

Not if most of it is machine generated. The machine would start eating its own shit. The nutrition it gets is from human-generated content.

> I don't understand the ethical framework for this decision at all.

The question is not one of ethics but that of incentives. People producing open source are incentivized in a certain way and it is abhorrent to them when that framework is violated. There needs to be a new license that explicitly forbids use for AI training. That may encourage folks to continue to contribute.

Saying people shouldn't create open source code because AI will learn from it, is like saying people shouldn't create art because AI will learn from it.

In both cases I get the frustration - it feels horrible to see something you created be used in a way you think is harmful and wrong! - but the world would be a worse place without art or open source.