The reason, Heinz argues, is that the language learner is not able to successfully distinguish between the two possibilities (Examples (17) and (18)), and is therefore biased towards learning a grammar in which the problematic expression 'σσ' uniquely maps onto a single substring of the grammar.
the learner is intrinsically biased toward neighborhood-distinctiveness, and for this reason languages that do not obey this requirement are not learnable, and hence, nonexistent.