WPcrack is only providing links for various themes, plugins & scripts which are licensed under GPL (GNU General Public License) and hence you can use them on as many websites you want and use it the way you want- modified or non-modified.
We are just providing you files. Please remember that if you want technical support for these files, you can purchase it separately from original developer of these scripts. This will help them financially for further development as well.
All the products offered in this website are licensed under GNU GPL License. Hence you do not need any license keys to use them in as many of your projects you need. These plugins and themes only require license keys to get automatic updates from their authors.