In fact, Zeller could do this every time he catches the ball - see if there are any scoring opportunities, and be aggressive if he sees something.
The majority of offense that teams run are not specific plays run for one particular person, but rather they run offensive sets, where they work the ball around to generate movement and find an open shot or exploit a defensive weakness or mishap. Are plays run for specific players? Absolutely, such as perhaps an out of bounds play, end of game situations, or if a player is on fire and you're trying to feed him the ball in the most favorable positions.
So as it pertains to the Bobcats, I'm sure you've seen us run our basic offensive set - Kemba goes down one side of the court, passes to the PF at the top of the key, with the wings coming off of screens to get the swing pass on the sides. They then continue through their series of picks and off ball movements, and continue to pass the ball around until they see a mismatch or other scoring opportunity. There are other offensive sets, secondary break offenses, zone offenses, two man games, etc. but the basic premise remains.
Josh and Zeller play the same position in the offensive set. So its not a matter of the coaches not putting Zeller in position to score, or him not getting it. The reason why Zeller sets up so high isn't just to take long jumpers, its for better spacing, so that there is room for someone to cut across the lane, post up, back screen, etc. Putting the swing man any closer and you'll clog the lane unnecessarily.
But looking back at Josh - there are no plays run for him to pump fake a 30 footer, dribble behind his back, then throw a no look between his leg lob for a Hendo dunk. What he has is a high basketball IQ, and is able to see how a defense is playing, understanding how to exploit it by his passing/ball handling/shooting abilities. If he sees a drive for a nice layup or backdoor pass, then he has the freedom to take it. Otherwise, keep the ball moving for the rest of the set.
When Zeller gets it high, he often turns to dump the ball off right away, normally to a guy cutting back across him or while screening the cutter. But he also doesn't have to hesitate to shoot 20 footers when left open. Even Dell comments all the time how Zeller should take a rhythm dribble to get a couple feet closer, or to pump fake to drive to his sweet spot.
And the chance to attack a clear lane is always left open for Zeller when he sees it. But we often see him taking a quick first step into the lane, only to get swamp feet and not able to get off the ground, resulting in an embarrassing block and getting tossed to the floor like a 7 foot rag doll. Its a matter of learning NBA defenses, reading it quicker, getting a better feel for the speed and strength of the game, and using your own athletic gifts to get easy shots. At this point, Zeller doesn't see the floor the same as Josh, which he can only attain with more experience and observation, and he also can't execute the plays equally without the proper strength and needing more practice on how to create opportunities.
I do agree with you that he should get post up opportunities down low a couple times, as I think he moves really well on the block and has good touch around the basket. Seeing the ball go in more often will help his confidence, and maybe get him more into attack mode. But coaches can't turn the light on for players, they can only help them see it for themselves. And if the best chance to get there is by observing someone better while on the bench, then so be it.