This sounds like a fair question, and it might be (including from a legal standpoint - IANAL), but the way I figure it LinkedIn will be constantly experimenting

This sounds like a fair question, and it might be (including from a legal standpoint - IANAL), but the way I figure it LinkedIn will be constantly experimenting with their recommendations anyway, whether those results are shared for research purposes or not, and they're not going to ask your permission to do that. It's what you sign up for when you join the site. And it's the same with other social media. Perhaps it seems more seriously because it's LinkedIn and it's a "professional" network. I'm not really defending them: it's more that I've always taken the view that I am responsible for my career and, whilst LinkedIn's tools and recommendations might be helpful, they're also not definitive, and I'm not going to hold them responsible for either opportunities that don't work out or for failing to show me all possible opportunities that might be a good fit.