> and the shields should be discarded every time

Maybe I'm missing something, but the page literally has a section titled "Clean and reuse your face shield" and lists the chemicals that should be used to clean it.

The doctor or nurse probably won't be able to do that process everytime they use the mask. Like other reusable hospital supplies it will be collected somewhere to be cleaned in bulk. The point above is that so many will be used in a short span of time that even a 24 or 12 hour cleaning rotation will not keep up without large numbers being available.

Mask => Shield

(Like many, I think I've got "masks" on the brain...)

this should only be done when you're so screwed that you don't have clean ones available. This is literally war-time thinking. It's also completely impractical to do at hospital scale, but if it's your only option...

Thats because demand is so high and supply so short that they are being re-used.