possible inconsistency on foreign key definition

hello - i am trying to wrap my head around foreign keys in 10.6. the following behavior does not make sense to me. This code block works just fine:

CREATE TABLE `myParent` (
`myKeyValue` varchar(10) NOT NULL,
KEY  (`myKeyValue`)
) ENGINE=INNODB;

CREATE TABLE `myChild` (
`myKeyValue` varchar(10) NOT NULL,
                      /*          KEY `myParent_to_myChild` (`myKeyValue`),   */
FOREIGN KEY `myParent_to_myChild` (`myKeyValue`) REFERENCES `myParent`(`myKeyValue`) ON DELETE CASCADE ON UPDATE CASCADE
) ENGINE=INNODB;

please notice i did not specify a key for the child.

however, when i create a child key and attempt to drop the child key, apparently i am not allowed to do so:

CREATE TABLE `myParent` (
 `myKeyValue` varchar(10) NOT NULL,
 KEY   (`myKeyValue`)
) ENGINE=INNODB;

CREATE TABLE `myChild` (
`myKeyValue` varchar(10) NOT NULL,
                        KEY `myParent_to_myChild` (`myKeyValue`),
FOREIGN KEY `myParent_to_myChild` (`myKeyValue`) REFERENCES `myParent`(`myKeyValue`) ON DELETE CASCADE ON UPDATE CASCADE
) ENGINE=INNODB;

ALTER TABLE `myChild` DROP KEY `myParent_to_myChild` ;

ERROR 1553 (HY000) at line 20: Cannot drop index 'myParent_to_myChild': needed in a foreign key constraint

why is it in the first example, no key is required, but in the second example, it is required?