Some clarification on why this is happening: When you aim quickly (i.e. time from button down to button up is very short) the system strongly prefers champions. We do this so that in fights you can more confidently target champions and not pick up other objects like minions or blastcones. Do you have any suggestions on how to improve this for your use case?
For blastcones, as others have pointed out you can use the minion/tower button (I think the tower button changes to a plant). For dragon/baron, your smite will autotarget the objective if it is low enough for you to kill it with smite. That's a little tricky, we considered making it always target objectives in range, but we want ran into problems where you were fighting around an objective and wanted to kill someone with smite and instead it would smite a high health/full health objective.