I think its because the SqlCommandBuilder cannot create insert/update commands if there is a join in the SQL.
This would mean, after adding the join to your Adapter, you would have lost the InsertCommand and UpdateCommand and therefore the ability to add/change your data.
http://msdn.microsoft.com/en-us/library/tf579hcz(VS.71).aspxI don't know of any SIMPLE way around it.