They're pushing to be able to force debit cards to be run over the debit networks, not Visa/MC. While most "debit" transactions need a PIN, it is possible to run smaller transactions over some debit networks without one. The distinction also isn't too relevant for cards from major banks either, as they cost merchants the same either way.

As for credit cards, merchants care about those less. Especially since being able to surcharge for those solves that "problem".