Or maybe, just maybe, Iran is actually doing what any country would do... defend its territory.
Things are never as simple as we'd like to believe... Makes for rough going from the "black or white", "with us or again' us" crowd. So much easier to yell "Kidnapping" and run around like a headless chicken. Makes it easier to justify a bit of bombing, too.
From the Guardian :

Because the two countries have not agreed on updated charts, that means there is no universal agreement on exactly where the border line runs.

If the seizure occurred near the mouth of the Shatt al-Arab - which is likely - the issue becomes even more complicated because Iraq and Iran have never agreed on each others' claim to Gulf waters near the mouth of the waterway.

Without such an agreement, international law requires countries not to extend their territorial waters ``beyond the median line with neighboring states,'' said Martin Pratt of the University of Durham in Britain.

But defining that line is difficult because of conflicting claims to rock formations, sandbars and barrier islands in the shallow waters of the northern Gulf, Pratt said.

As a result, there may be ``legitimate grounds for arguing for a different definition'' of those median lines, Pratt said.