In reply to glaramara:
This summer i climbed the Zmutt Ridge. I bivouacked on the ridge at 3600-3700 metres (because the Hornlihutte was too expensive and the Schoenbiel hutte was too far). I'm almost sure that this rule you mention is valid only near the hut.
I also remember that near the Hornlihutte there was signs which stated that bivouacking was forbidden UNDER a certain altitude (something like 3400m), so this means that bivouac on the routes are legal.
It's astonishing how in Zermatt all is too expensive, the hut, the cable car, the parking.. even buying something too eat at the supermarket! But the swiss know they can do it, because people will always go there from all over the world spending all the money they will require. When i was there i noticed lots of japanese, russians and arabian, and european from all countries too!