Can someone please explain why some champs (Fid, Kennen, Morg) are able to channel their ults while using Zhonya's?
That would be because those abilities AREN'T CHANNELS.
Fiddlestick's ultimate has a brief channel (which Zhonya's will interrupt) followed by a blink and a PBAOE effect which deals damage around him.
Morgana's ultimate attaches a tether to nearby champions, dealing damage and slowing when applied. If they stay within a certain radius of Morgana until the tether duration ends, they will be stunned and damage will be reapplied. At no point in this ability is there a channel.
Kennen's ultimate is a PBAOE effect which deals damage and applies stacks of his passive to everyone within its range. At no point in this ability is there a channel.
This seems kind of retarded to me. There is literally no counterplay to it other than 'stand in the ult'.
Actually, the counterplay is "walk out of the ult".
It's a simple mistake, but standing completely stationary in their ultimates is a dumb thing to do and I would advise you stop doing so.