I Gain Access to ALL of the Source Code? ~ Even the Pro Version?

That's right! We are firmly against locking the source code of any application. Like WordPress itself, both s2Member and s2Member Pro are released under a GPL license. You are free to modify and adapt these applications to meet your needs, or the needs of your clients. The source code for the s2Member Framework (i.e., the free version) is publicly available at WordPress.org and at GitHub.

When you purchase s2Member Pro, you get the source code for that also. ~ Hooray!