Mary,

DEFAULT can hardly be considered as a constraint, it is simply an atribute of a column. Constraints are used to *enforce* some rules that can not be overridden by users. DEFAULT does not enforce anything, it simply provides a default value if it was not explicitelly set in the insert. So I think they've put it in the data dictionary exactly in the view it belongs to - DBA_TAB_COLUMNS.

If you've actually found DEFAULT to be considered as a constraint somewhere in Oracle documentation, I'd call this more like a documentation bug...