Script Cutting

Having spent the year attending as many theatre productions as I possibly can, I have noticed the reluctance of community theatres to cut scripts. For some plays this isn't an issue, however there have been a number of plays that go over 2 and a half hours where some of the dialogue seems unnecessary. Just out of curiousity, why are community theatres so reluctant to cut scripts? Is it from fear of removing important story lines, dramatic or comedic moments? Is it a belief that the script and playwright must be right? Or is it a legal issue of copyright? TaylorM