The Problem:
Code: /admin/datadict-mssqlnative.inc.php, function DropColumnSQL.
1) Since the script is not actively executing, the query "select name from sys.default_constraints WHERE object_name(parent_object_id) = '" ... returns no rows.
2) Therefore, no ALTER TABLE DROP CONSTRAINT statements get generated, because the object being queried doesn't exist.

Solution (SQL Server-Specific):
1) Decide on a column default constraint naming convention. I use: [df_table_column], i.e. [df_mantis_bug_file_table_date_added]
2) In the CREATE TABLE statement, name the constraint for the column:
[column] {type} {NULL/NOT NULL} CONSTRAINT [df_table_column] DEFAULT {default}
3) Then, when you need to drop the column later on, you can issue:
ALTER TABLE [table] DROP CONSTRAINT [df_table_column];
ALTER TABLE [table] DROP COLUMN [column];

This way, the same script will work when executed immediately, or copied and executed later.