Why does Time#to_s(:db) force UTC?

I see in the original commit for Time#to_s in TimeWithZone (https://
github.com/rails/rails/commit/
022d9f7ce6d1237c4103a2aed561220e1c0f4dcc) to_s(:db) intentionally
forces UTC; conceptually, is there a reason for this? I would expect
it to respect ActiveRecord::Base.default_timezone.

I would file a bug if it weren't explicitly documented, so there is
some obvious intent. Is there a case where you would want to_s(:db) to
use UTC instead of ::default_timezone ?

-Woody