I also find this a bit irritating. It's bad enough hotels putting a hold on your card then charging your card proper when you check out, then leaving the hold in place for a week. However I feel United (or any airline) don't need to do this (Granted United claims that it's the CC's fault). What if this happened whenever you used your CC with every retailer?

The first time this happened to me (UK credit card), I was concerned that some place or someone might have overcharged me or worse might be using my card fraudulently. This concern was heightened by the fact that I booked (and quite often do) my ticket just before travel and went on to use the card abroad at many different /unfamiliar places. I rang up my card just and was reassured when I found out that is was UA still had a hold on my card. Now I'm more aware of the situation.