I'm not really sure what the argument is for letting these researchers make users of these platforms non-consenting participants in their studies? Is it even possible to opt out?
Nope :D The EU has determined that you WILL be a part of these studies!
Well, I am far more concerned with a secretive megacorp shaping public discourse. The requested transparency is a means to assess impartiality or bias of such platforms.
Yes, there is an opt out, make your profile private.