How do you know an LLM provider does not kill puppies every time you send a prompt longer than 14 words?

Because they have no incentive as a company to do that? But do have a strong incentive to learn from user input. (Most cutting edge features are developed private - very valuable to get that into your tool)

And getting the data is not hard, they already have it. Risky is indeed a bit making use of that data, as that requires at least some humans (as potential whistleblowers). But you don't even have to tell them, where the data came from.

Whether they do it? No idea, I assume not, but I see a risk.