I always had that same question in my head when i looked at some various sno-cross sleds. I finally asked a racer why they did that.....
I think it's a personal choice actually. Trail sleds usually have them so they stay flatter in corners. Now, sno-cross racers need to stay flat in the corners too, but if they set up thier shocks/springs correctly, then some opt to go without it. They usually have very stiff springs and strong valving, so they may prefer no sway bar.
I say this because when a racer is skipping along the tippy tops of many many NASTY moguls and jumps, having a sway bar may actually hinder thier performance if they hit the top of an uneven bump causing the sled to kick to it's side unwillingly. If a racer in the same situation had NO sway bar, but properly set up springs/shocks then only ONE side of the front suspension will absorb that NASTY bump. And yet, it will still corner well because the shocks/springs are so stiff that there is little tip-in. Normal trail machines need a smooth and graceful ride, so shocks and springs are relatively soft which then requires a sway bar for safety and driveability. Again.....this is my opinion from what i have learned from others.
I once drove an old Polaris 340 with no sway bar and it leaned in the corners as if i was trying to skateboard on a trampoline! Personally, i think it's up to the driver and which way they feel more comfortable riding.
