Because when you put them together, performing those scenes and/or monologues—regardless of whether it's called a "scene night" or a "showcase" (etc.)—is equivalent to performing a play. The only real difference between a 90-minute showcase with 15 different scenes and a full-length play is that there may be multiple authors; it's still copyrighted work being performed in the presence of an audience. Purchasing a script for each participant (just as you'd do for any play production) along with the Single Scene-Monologue Showcase Royalty (i.e., a pro-rated royalty) ensures that authors are properly compensated for the hard work that went into creating the material you're performing.