If there are no stats, then DB2 might make the wrong decision about which access path to use. Also, the estimated cost cannot be accurate if there are no stats and are based on default assumptions about the number of rows.
It's probably not going to help you much to speculate about why DB2 does something when there are no valid statistics. I am not sure why you are not getting distribution stats. You only need to run them once after a reorg, and not every single time.
I would be curious if the following performs any better (with full runstats on both tables):
from Table_B B
where br_id = 'COMPANY'
and reasn_cd = '1'
and exists (select id from id_list_table A where id.A = id.B)