Agreed: such tests are not sufficient to test the pre- and post-conditions themselves, and are definitely happy-path focused. But such tests may be sufficient to test the happy-path code itself (which the in-code checks are also testing). It would be ridiculous to have an entire test suite with no assertions for these reasons, but it's not ridiculous (in isolation) to see some unit tests with no assertions. In theory -- and this is not advice: theory and practice are different -- but in theory, if you've absolutely air-tightly tested the "happy path", then you've basically proven no other paths occur (short of cosmic rays).

But you could actually make this same argument about unit tests with assertions: do you have any test that those assertions are written correctly? Do you unit test your unit tests? Pre- and post-conditions living "in situ" are the equivalent thing to "control" and test assertions (respectively) living in your unit tests. Would you be more comfortable if the in-situ checks were simply cut/pasted into the unit test? I wouldn't be!