The military is known for its rules. There are books upon books filled with them. But even when there's no official documentation to back them up, troops adhere to rules laid out before them (usually). No unofficial rule is followed by as many troops as not walking on grass.
It's so prevalent in military culture that most NCOs don't even know why they're yelling at a private for walking on grass — they just know that first sergeant is looking.
To any civilian or new recruit, it's mind-blowing. Troops will do PT on the grass in the morning but once they're told to shower for work call, they're not allowed back on the grass until the following day (unless they're cutting it).
But why? A few footsteps aren't going to hurt anything.
If Sgt. Maj. of the Army Dan Dailey can come lead your unit's morning PT on the grass, chances are it's okay. (Photo by C. Todd Lopez)
To be completely straightforward: Your sergeant major doesn't give a rat's ass about the grass itself. The grass will still grow all over the world with or without "blood, bright red blood."
The restriction is symbolic and it's about not taking literal shortcuts. The idea is that if a troop takes a shortcut once, they'll see no problem cutting corners the next time.
Since military sidewalks are usually straight lines that intersect each other at 90-degree angles, a young private may save a half of a second by cutting through the grass. If enough troops cut that same corner, then the grass will die and become a path, thus destroying the need for the sidewalk to begin with.
Somewhere, there's a retired Sgt. Maj. knife-handing this photo.
Another reason for the rule is that it requires a level of attention to detail. If you're not capable of noticing that you're now walking on soft grass instead of the sweat-stained concrete, then this is very likely not the only ass-chewing you'll see in your career.
Your sergeant major
probably isn't a staunch environmentalist who's trying to preserve the sanctity of poor, innocent blades of grass. They and the NCOs below them have ten million more important things to do than to knife-hand the fool who's careless enough to do it — but they will. Stepping on the grass and spending the half-second required to stay on the pavement is symbolic of a troop's discipline.
H/T to the Senior NCOs at RallyPoint for clarifying this mystery.