Whoops, I forgot that "must not" means something strikt in english. This was my fault hehe
I meant: You don't need to turn it off, but you can.
The tabs get colored as long as you enable that option.
Standard ESO is only coloring the chat tab for the "whisper messages" if you receive a new whisper message.
Edit:
And the inactivity time is a kind of prevention method so the chat tabs won't change to another one as you are currently reading the last incoming message.
You can set them to e.g. 5 seconds for guild messages or more frequent chat tabs and to something like 20 seconds for the /say or /zone chat. Depends on where you are more active.
The chat tabs won't change then if they were changed from last incoming mesasge and you are currently reading the last message or answering (or are on your way to answer).
Otherwise each new message would cycle the chat tabs and you would just see them flapping away after each other without beeing able to read a single word
