The point was to show tree level and visibility. Almost all of Tignes is above tree line, so with any little snow fall or a bit of cloud, the visibility goes way down. That's not too bad if you're used to it, but if you're not used to it, it can be really off-putting. The only part of Tignes below tree line is Tignes le Brevieres, which only has a couple of pistes and the trees are not skiable (unless you like cliffs and rock faces).
From what I've seen skiing in North America and in Europe with people from North America, most of them are not used to skiing above tree line in bad visibility. I just put up the webcam shots to show people what the visibility is like on non-bluebird days in the two resorts. Today is another cloudy/snowy day in Tignes, so the webcams give a pretty similar picture as before.
Val D'Isere is a better place to be on a low visibility day then St. Anton, which I believe has no tree skiing. As others have pointed out, there is tree skiing off the La Daille lift. I have 35+ days at Espace Killy over the past 6 years and have never missed a day because of bad light. This year, I skied 8 days and it snowed 6 but no issues. The resort is big and you can have a total whiteout in one part of the mountain with decent visibility elsewhere. I quite like the snow days because you have fewer people on the slopes.