When you say "can be internally valid" what do you mean? What about interactions from repeated treatments? I mean, correlation can equal causation, too. But that's a pretty weak standard to meet.
Also, how do you know the selection bias does not create non-causal explanations for observed dependence? For example, in case control studies you select based on the child of the outcome:
T -> Y -> S, with unobserved U1 being a parent of T, and U2 being a parent of Y (U1, U2 possibly dependent creating unobserved confounding).
If we select on S FIRST, and THEN try to randomize T (conditioning and do(.) do not commute), then we create a dependence between T and U2 due to "explaining away." Randomizing on T cuts the arc from U1 to T (good -- we get rid of some unobserved confounding), but does nothing about this new dependence between T and U2 introduced by the selection procedure.
When you say "can be internally valid" what do you mean?
http://en.wikipedia.org/wiki/Internal_validity
What about interactions from repeated treatments?
?
Also, how do you know the selection bias does not create non-causal explanations for observed dependence?
I don't understand your hypothetical. Could you give a concrete example?
http://www.johndcook.com/blog/2013/02/04/four-hours-of-concentration/
And since this is the Internet, and facts are involved, our gwern turns up there also.