Why does PostgreSQL not return null values when the condition is <> true

Question

I was confused behind the reasoning of the following:

SELECT * FROM table WHERE avalue is null

Returns x number of rows where \'avalue\' is

Answer 1

Every halfway decent RDBMS does it the same way, because it's correct.
I am quoting the Postgres manual here:

Ordinary comparison operators yield null (signifying "unknown"), not true or false, when either input is null. For example, 7 = NULL yields null, as does 7 <> NULL. When this behavior is not suitable, use the IS [ NOT ] DISTINCT FROM constructs:

expression IS DISTINCT FROM expression
expression IS NOT DISTINCT FROM expression

Note that these expressions perform a bit slower than simple expression <> expression comparison.

For boolean values there is also the simpler IS NOT [TRUE | FALSE].
To get what you expected in your second query, write:

SELECT * FROM table WHERE avalue IS NOT TRUE;

SQL Fiddle.