If a NULL were just a value in 3-valued logic, it could have been OK. However, that's not how it works out. Consider the following (that should be equivalent if NULL is just like Maybe in True-Maybe-False logic):

  SELECT null AND true;

  SELECT bool_and(column1) FROM (VALUES (null::bool),(true)) AS foo;

In PostgreSQL the first query produces NULL, while the second produces TRUE. Yet, it's also possible to get NULL as a result of aggregating values with bool_and:

  SELECT bool_and(column1) FROM (VALUES (null::bool),(null::bool)) AS foo;

And that's not how a 3-valued logic works.