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;
You are mixing two concepts here: 3VL and how most aggregates work in SQL: the drop NULL values before doing their work. That's why the first BOOL_AND example only sees one value, thus returning true.
