At large scale, it probably adds up. Remember that they also need to ensure backup, etc. Whether or not your code is being "used" or not, it has a cost.
Even GitHub has been struggling last year with the ramping amount of LLM produced code. I can imagine that it is a real issue.
They already encourage users to use 'agit' instead of doing 'physical' forks.
I imagine that the costs are sufficient enough to motivate them to follow through on this; on top of the ethical and legal concerns.
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Why don't you host your own git for those dormant projects? It's really easy, you don't even need a forge. Just a bare git repo on server via ssh and you off to the races.
I think their stance is fine; there are other providers if you don't agree with this.