Why? This is the same concept in any hot/humid conditions during the summer regardless of hemisphere. It's good advice even if you hear it during the winter.

People have different standards. I'm whining about the heat at 26 degrees, but it's not getting me out of mowing the lawn.

And not working during a hot winter day is comedy sketch territory. That snow isn't going to shovel itself!

...it's not dangerously hot if there is snow on the ground...

You've inverted the season, so now you need to invert the temp as well.

So don't... lie still and avoid generating heat in the snow?

The southern hemisphere has the same weather as the northern hemisphere, but with reversed seasonality. You might be thinking of the American subtropics.

Because it's freezing in the northern hemisphere right now, that's why.