Suppose a PP-complement could select a VP mother. The function f VP on this node is satisfied, but this in not due to any of its daughters. The function f VP on PP depends on the dominating VP for its interpretation. This implies, then, that upward function application violates inclusiveness, just like downward copying. Copying may apply recursively, since it involves the transmission of information.
Downward relations are relations of function application.
Since function application does not involve the transfer on information of one node to another, it cannot apply recursively. The previous two conclusion together explain why grammatical dependencies satisfy c-command. Thus, grammatical dependencies involve any number of steps up, but just one down.