Personally, I reckon the answer is probably... maybe. I don't know if stranglehold is quite the word I'd use, and I'm not sure they have that much of a concern with events outside of their own calendar, BUT they do exert a great deal of influence over running their events as they want to.
I have said before, and will bore people again with it, I do get fed up with the sniping that goes on between ASO and UCI. I do think ASO should back off, stay within it's business remit, and let UCI do it's job.... sad thing is, it's so damned questionable as to whether UCI is doing it's job. If they were, maybe ASO wouldn't feel they need to try and control its 'brand'.