Handling of tz-aware literals in non-tz-aware fields

Hello,

the issue can be show with this example:

piro=> SET TIMEZONE = 'America/New_York';
SET

piro=> select '1970-01-01T00:00:00+03:00'::timestamp;
timestamp
---------------------
1970-01-01 00:00:00
(1 row)

piro=> select '1970-01-01T00:00:00+03:00'::timestamptz::timestamp;
timestamp
---------------------
1969-12-31 16:00:00
(1 row)

I find surprising that an unknown literal containing a TZ-aware
timestamp has the tz info discarded (e.g. upon insertion in a
timestamp without time zone field), whereas the cast from tz-aware to
non-tz-aware performs a conversion. I find the second behaviour much
more reasonable.

Is there an explanation for the first behaviour?

Is the first behaviour documented?

Thank you very much,

-- Daniele