Checking For A Default Value Of A Parameter

I am using a query to grab information about the parameters of stored procedures. I am trying to determine if a parameter has a default value. I have tried adding a default value to the parameter of the stored procedure, but the returned value of params.has_default_value stays at zero. What is going on?

Here's the query to pull the stored procedure information:
SELECTprocs.name as ProcName,
params.name as ParameterName,
types.name as ParamType,
params.max_length,
params.precision,
params.scale,
params.is_output,
params.has_default_value
FROMsys.procedures procs
LEFT OUTER JOINsys.all_parameters params
ONprocs.object_id = params.object_id
LEFT OUTER JOINsys.types types
ONparams.system_type_id = types.system_type_id
ANDparams.user_type_id = types.user_type_id
WHEREprocs.is_ms_shipped = 0
ANDprocs.name = 'webservices_BENEFICIAL_USES_DM_SELECT'
ORDER BYprocname,
params.parameter_id


Here's the stored procedure:
ALTER PROCEDURE [dbo].[webservices_BENEFICIAL_USES_DM_SELECT]
-- Add the parameters for the stored procedure here
@DISPOSAL_AREA_NAME DISPOSAL_AREA_NAME_TYPE = 'a'
AS
BEGIN
-- SET NOCOUNT ON added to prevent extra result sets from
-- interfering with SELECT statements.
SET NOCOUNT ON;

-- Insert statements for procedure here
IF @DISPOSAL_AREA_NAME IS NULL
BEGIN
SELECT*
FROMBENEFICIAL_USES_DM
END
ELSE
BEGIN
SELECT*
FROMBENEFICIAL_USES_DM
WHEREDISPOSAL_AREA_NAME = @DISPOSAL_AREA_NAME
END

END