It was (if deliberately) a decision made by David Korn.
There are no requirements for this behavior produced by external forces - it does not require backward compatibility; there is no reason to adhere to standards (in this respect, POSIX sh was not yet standardized when ksh88 came out).
The only person who can give a definitive answer to the justification - if any - for unspecified behavior (behavior that does not implement not violating any documented semantics) is the author of these shells.
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