CQRS cross aggregate interaction + private state

If entities only have private state, how do you deal with interaction between aggregates?

e.g.

product = GetProduct(123);
order.AddProduct(product,3);

Lets for the sake of the argument that addproduct can not occur if product is marked as obsolete.

How would you deal with that?
What's the idea behind completely private state?
Why shouldn't order be able to read properties on product?

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