You don't need to purchase a license for the FTP Library. But the FTP Library contains examples you can use.
Scripting in the Professional Edition
You can create scripts to automate SmartFTP. SmartFTP uses the FTP Library. And you can use the FTP Library component through the SmartFTP object/application. A Pro License for SmartFTP is required for each user of the application/script you develop.
License for FTP Library
You can create your own applications which use the FTP Library. You can distribute your application and the users of your application do not need to have SmartFTP installed or they don't need a license for SmartFTP Pro.