Tie in point on harness - belay device

I do think that cross-loading is not the big issue when using the GriGri. For me the big thing is that the GriGri is twisted 90 degrees, in which angle the GriGri can not lock. If you do it in both the waist and belay-loop, the GriGri is turned 45 degrees.

I work in a gear shop and all the neighbouring gyms teach to clip through both the leg loops and waist belt. Very annoying! Many people do so because of some idea of redudancy to their 25kn belay loop. This completely ignores the fact that they are climbing with one biner and one belay device on one rope!

I know its wrong to clip the belay device carabiner through leg and waist loops instead of using the belay-loop, but could somebody explain (or give a link, because i can hardly find anything) why?

Use the belay loop! If you thread the biner through the leg and waist loop you risk cross-loading, or worse, tri-axial loading of the biner. Your biner holds maybe 23-28 kN when loaded along it's longitude, against only 7-9 kN when loaded laterally (cross-loaded). If loaded both ways it should be even less, and I would not consider whipping on it. You should by all means use the manufacturer's instructions, which are to use the belay loop. If it's there, use it.

Iif you're not sure read the Manual from either harness or belay device. There it will say that you should belay from the belay loop (and the belay loop only) (Atleast i've never seen a harness or belay device that has other instructions)