Observations:
-- Check either the HOA Declaration or the plats on file with the county. Do they show the parking strips to be common area? If so, this is a strong argument for the association being responsible for maintaining the mailbox structure (located on the common area).
-- Whether the individual owners are responsible for the structures, or the HOA is, will result in about the same cost to the owners.
-- I expect uniformity of appearance of the mailbox structures adds value to homes. If the HOA maintains the structures, chances are much greater that the appearance will be uniform.