Trying to divine exactly why anyone would be "surprised" by Iran doing what everyone assumes to be the obvious is nigh impossible.
Generally, though, assuming stupidity on the part of someone making a decision you don't agree with and/or don't understand is rash folly (and bad manners). Your reasoning and their reasoning is each subject to the uncertainties introduced by different factors (knowns and unknowns) and different weightings. If premises are not identical, conclusions are likely to be different.
Another possible explanation is "We do X, which gives them an excuse to do Y, which gives us the excuse to do Z". "Oh, but we ourselves don't really care about Y, and Z would be catastrophic for them. They must know that. They would never do Y." So you'd be surprised if they did in fact do Y, unless you thought of something beyond Z that would swing things back in their favour.
A great number of critics simply can't stop themselves from situating the US where they think it ought to be. When the US says it doesn't care much about something that some other countries really care about, it is pointless to insist that the US has to care about it.
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