The way around this is to use :reorder, but :reorder doesn't work the same way and can generate invalid SQL if you're prefetching included associations. So, you're left with :unscoped, but that's insanely insecure because it drops any foreign key scopes you had in place.
Likewise, it only normalizes identical order clauses if you use the exact same casing. This one is a bit more esoteric, but if your default_scope is to order by "id asc" and you reorder by "id ASC", Arel will produce "ORDER BY id asc, id ASC". If they match in case, however, it'll be normalized to a single clause.
I haven't found the time to attempt a proper fix to any of these problems yet.