. . . because they don't look like gunboats to me. The one shown in close up is bright blue, for a start- hardly a military paint job. Secondly its a closed-bow design with two people in it and no visible armaments. "Gunboats" suggests boats with guns in them. This looks more like a toy. Certainly the way they're being driven doesn't suggest hostile intentions- the US made a point of observing that they were in international waters and therefore every right to be there, it looks more like a bunch of rich twats on a yeehaa mission.
Secondly, how does the Pentagon know they were Iranian, let alone from the revolutionary guard? Maybe they spoke Persian or the radar indicated they came from the Iranian coastline but so fucking what? This is pretty weak stuff, even for the Pentagon- the people who brought us "Iranian-made" IEDs and Extraordinary Rendition.
CounterPunch also smells bullshit re: the Gulf of Hormuz incident- or was it the Straights of Tonkin?