You misunderstand. Sure, you can tell when you can place one when it's already ready. But in the 24 or some odd seconds between your last turret and your next one, you can't tell when the next one will be available, there's no timer (unless you're pro enough to count it off in your head). The skill is just greyed out until the next one becomes available, at which point you get the icon that you mentioned. Teemo, on the other hand, has a CD timer so that you are able to judge when you are able to place your next shroom, even if you've run out of available shrooms to place. Do you see what I mean?
1) Place a turret
2) ?????????(skill unavailable)
3) Ok, your turret is ready now.
I just think they should make that 24second interval visible to the summoner. It might improve his game-play, if only slightly.