Snap7 is distributed as a binary shared library with full source code under GNU Library or Lesser General Public License version 3.0 (LGPLv3)
Basically this means that you can distribute your commercial software linked with Snap7 without the requirement to distribute the source code of your application and without the requirement that your application be itself distributed under LGPL.
A small mention to the project or the author is however appreciated if you include it in your applications.
THERE IS NO WARRANTY FOR THE PROGRAM, TO THE EXTENT PERMITTED BY APPLICABLE LAW. EXCEPT WHEN OTHERWISE STATED IN WRITING THE COPYRIGHT HOLDERS AND/OR OTHER PARTIES PROVIDE THE PROGRAM "AS IS" WITHOUT WARRANTY OF ANY KIND, EITHER EXPRESSED OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. THE ENTIRE RISK AS TO THE QUALITY AND PERFORMANCE OF THE PROGRAM IS WITH YOU. SHOULD THE PROGRAM PROVE DEFECTIVE, YOU ASSUME THE COST OF ALL NECESSARY SERVICING, REPAIR OR CORRECTION.
IF ANYONE BELIEVES THAT, WITH THIS WEBSITE OR WITH SNAP7 PROJECT HAVE BEEN VIOLATED SOME COPYRIGHTS, PLEASE EMAIL US, AND ALL THE NECESSARY CHANGES WILL BE MADE.