They claim it is for safety reasons, although I'm not sure when/if I would be in a situation to where it would be an issue. They seem to be old fashioned and lean toward the "clean-cut" look. A mustache is allowed as long as it is neatly trimmed and doesn't go past the top lip. 
Safety? It sounds like B.S. to me. Any company where the management is regulating the shape of a mustache is a management team that is not spending enough time doing actual work. With nearly every business now facing some degree of global competition at the same time litigation and regulation have never been as intense, it is nothing but gross incompetence for any employer to sit around focusing on ways to regulate facial hair (it's even offense to believe they have the right to regulate it anyway). I mean this is something straight out of Dilbert!
Could you imagine this scenario?
Bill Johnson, CEO:
"Yes, last quarter our earnings sank to a new low because our poorly-designed and under-tested product resulted in the deaths of dozens of people, and the new regulations forcing us to drop our workforce to 50 full time employees means we'll never get it corrected, but you have to focus on the big picture. Of the male employees that we did manage to keep who insist on growing facial hair, they are now mandated to trim it into a geometically-perfect trapezoid of hair on their upper lip. This is the protractor, marker, and ruler I use to enforce compliance!"
