There's a certain logic here.

Greasing reduces the friction which keeps the stem aligned with the fork. This is easily compensated for by making it tighter, but that in turn increases the stress on the steerer.

We're in a litigious era, and their advice is safer (for the maker) because it reduces the risk of a ham fisted owner getting carried away and blaming them for any consequences. For my part, I disregard instructions and continue to use the basic tried and proven methods that served for a century or so.

I use a light grease or heavy oil as a corrosion barrier, and tighten "race tight" meaning only enough to reliably stay put in use, but loose enough to slip in a crash.

FWIW - I've seen all too many instances of heavy riders using a stiff grease or anti-seize product on the seat post, and ending up breaking the clamping ears as they try to get it tight enough not to slip. These days, when someone comes to me with a slipping post problem, I clean the post and seat tube to dry bare metal, apply grease to the inside of the seat tube 2" below the top. Slide in the post twisting to spread the grease as I insert it to depth, then align and tighten, being careful not to pull it back up. This yields dry metal or metal contact at the clamping area to prevent slippage, with grease below to prevent seizing.