"Going across these slopes is also NOT LNT hiking. Up, down or across, one leaves a visible scar."
True enough, Michael, but the same can be said for crossing a pristine snow slope or sand dune. In the first 2 cases the trace is transient. Wind, rain, sun, or winter snow will erase all trace of one's tracks in the mountains, be it on scree or snow. In any case, if travelling off trail in ranges like the Sierra or Cascades, leaving tracks in scree or snow is often unavoidable, whether one walks, runs, or crawls. The same cannot be said of desert environments and I, personally, would tread much more carefully there. Same goes for fragile vegetated areas anywhere.
Regarding frolicking on scree slopes, or snow slopes for that matter, it should be emphasized that before the frolicking begins one needs to very carefully check the run out for cliff bands, crevasses, rocky areas below a prospective playground. In the case of glissading in potentially crevassed areas, ALWAYS glissade standing up or better yet, don't glissade. My 2 cents.