If these residential locks are truely masterkeyed and you find this consistently true in specific neighborhoods or from specific builders send out flyers to every house in said neighborhood or built by said builder indicating that you have good reason to believe the homeowners lock is masterkeyed and can be opened by keys in the possesion of others besides the owners. Offer to check the locks for free (if you send the flyer you should already be virtually positive the problem exists) then re-key at your regular cost, or sell them better locks altogether. You could make a killing and solve a real security problem for lots of people all at the same time.