I'd assume that while selecting which forums should be displayed in the above feeds we want to select most forums except for a few, like private/hidden forums.

If so it would require much less clicking if the editing option was to select forums we DO NOT want to display instead of those we want to display.

With current option those forums take a whole height of my monitor screen...lol

 

I believe it is this way, since it keeps consistency to the field type (which is a node form helper), with other parts of the suite that it would be easier to include what you want vs selecting what you don't want.

I fully understand. I'm just trying to go for convenience.

i know this isn't ideal, but you could use some css trickery in custom.css to probably reduce the height/fontsize of the tokens, so its not taking up the height of your monitor

No big deal, really. I just had to do that edit once. It'd be rather for future users.