Restriction On Scalar Subqueries

I would like to pass my 1Z0-047 certification, but I don't understand the limitation on the scalar subqueries, especially for the having clause.

Here is my scalar subquery because it returns only one value.

CODEselect avg(list_price)
from product_information

I use it in a having clause as a scalar subquery and it works

CODEselect status ,avg(list_price)
from product_information
group by status
having (select avg(list_price)
from product_information)
>= avg(list_price);

but it is documented that it can't works :

QUOTE There are also important restrictions on scalar subqueries. Scalar subqueries can�t be used for:
Default values for columns
RETURNING clauses
Hash expressions for clusters
Functional index expressions
CHECK constraints on columns
WHEN condition of triggers
GROUP BY and HAVING clauses
START WITH and CONNECT BY clauses

I probably don't understand the limitation .