WC plugins are 100% GPL. If you take a delivery of a plugin, you get all of the source code, and have all of the freedoms to use / modify etc. as WP itself.
The whole situation with large amounts of "premium" plugins that apparently are GPL for the most part (or at least should be) but don't like to talk about it, include all kinds of license activation features and are split into a separate "free" version with reduced feature set and advertisements splattered all over is a little strange. It's also very off putting when you're used to the typical way open source is developed and distributed.