Why are JSON extraction functions STABLE and not IMMUTABLE?

In current 9.3, I see:

$ select p.proname, p.provolatile from pg_proc p join pg_namespace n on p.pronamespace = n.oid where n.nspname = 'pg_catalog' and p.proname ~ 'json';
proname | provolatile
---------------------------+-------------
json_in | s
json_out | i
json_recv | s
json_send | s
array_to_json | s
array_to_json | s
row_to_json | s
row_to_json | s
json_agg_transfn | i
json_agg_finalfn | i
json_agg | i
to_json | s
json_object_field | s
json_object_field_text | s
json_array_element | s
json_array_element_text | s
json_extract_path | s
json_extract_path_op | s
json_extract_path_text | s
json_extract_path_text_op | s
json_array_elements | s
json_array_length | s
json_object_keys | s
json_each | s
json_each_text | s
json_populate_record | s
json_populate_recordset | s
(27 rows)

Is there any particular reason extract functions
(object_field/array_element/...) can't be immutable?

I can't readily imagine a situation where output of these functions would
change for different queries.

Best regards,

depesz

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