Because it's the right thing to do and I doubt it really has much influence on liability unless you say something like "we're not working with you because you're a black woman".
I am afraid you failed to read the fine print ;) I think the average legalese boilerplate is along the following lines: All characters and events depicted in this film are entirely fictitious. Any similarity to actual events or persons, living or dead, is purely coincidental.
All "episodes of Galaxy Quest are true historical documentaries".
Classic.