Gasket genius: We need you now!

I bought a master cylinder once from I believe Master Power Brakes, and it had a label that said pretty plainly: "Do Not Use DOT 5 fluid". I have since seen others tagged that way as well. Any idea as to why?
The only reason I could possibly imagine is, they're afraid someone will mix the two fluids and cause them a warranty situation. Other than that, I got nothin.