I'm not sure if open weights are immune to being compromised by ads anyway, they can't serve pay-per-impression ads on the output side, but there's nothing stopping the creator from accepting funding in exchange for biasing the training one way or another.
Coming soon: Foobar-600B, a new SOTA open weight model kindly sponsored by Coca Cola, Exxon Mobil and the Heritage Foundation. Please pay no attention to the men behind the curtain.
Without that though? Our ability to manipulate LLMs is so shaky I would be really surprised if anyone managed to pull off this kind of model manipulation and have it remain undetected.
If you're tucked in right behind the absolute frontier models, the economics change completely
[edit] I was a little unfair -- lack of access to training data is a bit of an issue (perhaps moreso for analysis than for for actual use, considering what it takes to train these models). I'm thankful that some of them are also distributed as base models, which should be relatively unbiased compared to what happens later during finetuning.
Similarly, I’m wondering when huggingface is going to need to start showing returns and starts putting ads into transformers etc.