True. Both variants are legal, and most people won't ever notice. I stumbled across this while writing a test case for a transaction helper that sets/restores search_path before committing. The test was to simply compare the string values of `SHOW search_path;` before `BEGIN TRANSACTION;` and after `COMMIT;`.
It's a non-issue, really, but since there's a patch and I cannot come up with a more common use case that would depend on the use of just-comma separators in the default value, I'd say it's more of a question of "why not" instead of "why", isn't it?