The same origin paradigm: the origin can do whatever it wants with data it has access to.

Notably this includes flinging whatever data it can see to anywhere on the network.

So as a user you have to be very careful about what data you put in each origin.

This has a chilling effect up the stack of what data each entity is willing to share in the first place.

That leads to data hoarding, and the entity that hordes the most data the best gets all of the user activity.