I figured out why.
In the plugin's lib.php file, there is actually a "default" designation for roleid=5, which is Moodle's good ole "student" role id.
I changed the 5 to 15, which is the roleid my custom role exists as, and guess what?
IT WORKED.
The author, or whomever handles Sign Up should rework their plugin so it registers a change in default role to assign to registered users ON THE CONFIGURATION PAGE, not in the plugin's base code.
Hope I could help someone.