And the owner could have sold it and stolen it back in in the many months he have bought it. I guess if you manage to block the device in the store itself, they will unblock it for you - but it would defeat the purpose in this case.
It's a separate question whether the customer should be able to disable Activation Lock before it gets tripped, which would prevent the computer from bricking itself. Apple makes that choice for the customer because they don't fully respect your ownership of the device that they made.