AS operator and subselect result names: PostgreSQL vs. Oracle

Hello,

I would like to ask the about the following...

PostgreSQL allows tables resulting from subselects to be renamed with
an optional AS keyword whereas Oracle 9 will report an error whenever
a table is renamed with the AS keyword. Furthermore, in PostgreSQL
when the result of a subselect is referenced in an outer select
it is required that the subselect result be named, whereas this
is not true in Oracle. I wonder what standard SQL has to say
about these two issues. In particular:

1. Does standard SQL allow an optional AS keyword for (re/)naming
   tables including those resulting from subselects.

and

2 Why must a subselect whose fields are referenced in an outer query
  be explicitly named in PostgreSQL when it is not necessary in Oracle.

Thanks,

Neil