Why do the streams in the "Cool-Streams" list default to MP3 for streams that are available in Vorbis too? Vorbis offers better quality, less bandwidth and is open and free. So why does Amarok prefer MP3?

Well, why not list those streams that are available as Vorbis too, so that we can change it?

I've hunted them down one by one 

Virgin Radio: http://network.virginradio.co.uk/core/a ... rvice=vrbb
DnbRadio: http://dnbradio.com/ogg.m3u
Frequence 3: http://streams.frequence3.net/ogg-112.m3u
Nectarine Demoscene Radio: http://nectarine.sik.fi:8002/high.ogg.m3u

This one does not provide a Vorbis stream, but the stream is broken in Amarok:
Philosomatika: needs: http://www.shoutcast.com/sbin/shoutcast ... lename.pls

These also don't provide Vorbis, but there's a higher quality stream available while Amarok lists the lower quality one:
Pure DJ: http://www.puredj.com/etc/pls/192K.pls
Radio Paradise: http://www.radioparadise.com/musiclinks/rp_128aac-1.m3u

Question: How do I edit my local list of those? There's no "Edit" entry when I right click on a stream in the playlist

Just add them as new stations - that will make them easier for you to always find anyway. Cool Streams is great, but it's a starting point.