Sorry if this is very simple but when I create a user, it creates a tablespace identically named for this new user, correct? If that is correct, why does Oracle orphan the tablespace in the database after a user has been dropped using the cascade option?
If "it" is CREATE USER statement, your claim is incorrect. The only thing you may do is to specify already existing tablespace as DEFAULT/TEMPORARY one in relevant clauses.
As tablespace does not belong to any specific user, it is not dropped by DROP USER statement with CASCADE clause.
All SQL statements are described in SQL Language Reference, which is available with other Oracle documentation books e.g. online on http://tahiti.oracle.com/.
Please, consult it for more details about them.