To the extent that this linear constraint on copying is stipulated, an important part of the descriptive generalization that some PIs require a preceding overt licenser is, in effect, restated.
Besides avoiding all conceptual problems, the appeal to linear precedence accounts in a unified way for all the data, including inverse-scope structures like (30): even in those cases, the licenser precedes any N, even though it does not c-command it.
At LF, the licenser is the whole fronted PP; but the morphological requirement cannot be met by this complex phrase, because it is terminal nodes that undergo vocabulary interpretation.
My (tentative) answer is that the relevant PIs are "special" in that they cannot be spelled out unless information about their licenser is provided, and this information can only be copied from a linearly preceding [M.
The observed precedence requirement between licenser and PI is formalized, if not properly explained, by the assumption that copying between elements of the linearized string requires precedence:
What makes it possible is the copying procedure illustrated in (43)-(44) above, whereby a terminal interpreted as PI receives a purely morphological piece of information in the form of the feature [operator], copied by a preceding licenser according to (45).
Indeed, the inclusion of [operator] among the input features of a vocabulary item is the crucial property that characterizes those PIs that require an overt and preceding licenser.
The added [operator] feature takes on the same value as its source, namely, the value of the licenser.
However, free-choice any is dependent on the semantic context alone: provided the sentence is compatible with a generic, nonepisodic interpretation (unlike [51c], for example), free-choice any is licensed, without requiring a spelled-out licenser nor a particular configuration.
Therefore, only polarity any needs copying of [operator] as a repair strategy, and consequently an overt and preceding licenser.
It is correctly licensed in the source VP, but no licenser appears in the clause after but for the copied VP.
Since they do not enter morphological interpretation, there is no need for an overt licenser, or indeed for a licenser at all.