you must identify to the nick you want to drop if you are not a services admin, therefore the drop command is already password protected.
If you are a services admin, there is no need to have the password to drop other users nicks. Not to mention that to drop others nicks you must be opered and identified to nickserv as a services administrator. Therefore the drop command is already password protected.
Thus it's pointless to require yet another password to drop a nick.