Why does the READPAST hint cause indexed views to be ignored?
Reusing the example table and indexed view from my article Another Reason to Use NOEXPAND hints in Enterprise Edition:

CREATE TABLE dbo.T
(
    col1 integer NOT NULL
);
GO
INSERT dbo.T WITH (TABLOCKX)
    (col1)
SELECT 
    SV.number
FROM master.dbo.spt_values AS SV
WHERE 
    SV.type = N'P';
GO
CREATE VIEW dbo.VT
WITH SCHEMABINDING
AS
SELECT T.col1 
FROM dbo.T AS T;


Repro

This query matches the indexed view (albeit with a redundant aggregate):

SELECT DISTINCT
    VT.col1 
FROM dbo.VT AS VT;




Adding a READPAST hint results in accessing the base table:

SELECT DISTINCT
    VT.col1 
FROM dbo.VT AS VT 
    WITH (READPAST);




Explanation

The READPAST hint is semantic-affecting. The optimizer resists rewriting queries such that the results change. To illustrate:

The following query executes without problems:

SELECT DISTINCT
    VT.col1 
FROM dbo.VT AS VT 
    WITH (READPAST);


However:

SELECT DISTINCT
    VT.col1 
FROM dbo.VT AS VT 
    WITH (READPAST)
OPTION 
    (TABLE HINT (VT, FORCESCAN));


Produces the error:


Msg 8722, Level 16, State 1, Line 42
Cannot execute query.
Semantic affecting hint 'readpast' appears in the 'WITH' clause of object 'VT'
but not in the corresponding 'TABLE HINT' clause.
Change the OPTION (TABLE HINTS...) clause so the semantic affecting hints
match the WITH clause.


When you reference the indexed view without the NOEXPAND hint, the view is expanded (before compilation and optimization begins) to reference the underlying objects instead. Later in the process, the optimizer may consider matching the query tree back to an indexed view, in whole or in part.

When READPAST is used without NOEXPAND, the hint propagates to the base table, preventing view matching (different semantics).

With NOEXPAND, the hint applies to the view directly, so there is no problem.