I disagree. I don't see the harm caused by leaving objective-c on these questions. If they involve the Objective-C language in some way, the tag would seem to be appropriate, even if the question isn't about a specific aspect of the language.
It is inconsistently used, but I don't think that causes much of a problem. If people want to find iOS questions, they follow the iphone and/or ios tags, and Mac developers go to cocoa. Whether or not objective-c is present doesn't change the visibility of the question much in my experience, although it may help clarify cases where someone explicitly doesn't want to use a C or C++ approach to a problem. I wouldn't waste your time with retagging tens of thousands of questions one way or the other for minimal gain.
However, if you want to see a tag being misused in the iOS areas, look no further than xcode. That mistagging really grinds my gears, and I often edit that out. Man, do I hate seeing questions like "how do I write image downloading in xcode?"