Why the WHERE clause is not pushed down in the view's query?
This query:
SELECT DISTINCT ON(recipient) * FROM messages
LEFT JOIN identities ON messages.recipient = identities.name
WHERE messages.timestamp BETWEEN timeA AND timeB
ORDER BY recipient, timestamp DESC;

says:

For all messages between timeA and timeB, find the recipients and for every recipient, find one message (the latest in between timeA and timeB).


This query (which you would get if you use the view):
SELECT *
FROM 
  ( SELECT DISTINCT ON(recipient) * FROM messages
    LEFT JOIN identities ON messages.recipient = identities.name
    ORDER BY recipient, timestamp DESC
  ) AS myView
WHERE timestamp BETWEEN timeA AND timeB;

says:

For all messages, find the recipients and for every recipient, find one message, (the latest in all times) and then, show only those messages that are between timeA and timeB.


As a result, the first query will show messages that are between time A and
B which the second query will not show (because there may be one or more messages for the same recipient, later than time B).
So, the queries are logically different and the condition cannot (and shouldn't) be pushed down for your view.
If you want to have parameters passed to your view, look the two answers (by @a_horse_with_no_name and @Erwin Brandstetter) in this question for how to use a set returning function: Pass In “WHERE” parameters to PostgreSQL View?