Why? Because it's possible -- not saying it's true yet, mind you -- that just maybe the guards thought Watts was dangerous. And by Watts' own admission he first broke protocol for dealing with border guards (by getting out of the car), then violated a direct order by a law enforcement officer. So to leap without further evidence to the conclusion that the officers were being "gigantic dicks given excessive amounts of power" requires that you don't care what the guards thought was happening.
It's interesting to compare this incident to the "flying imams" event a few years back. I've read the police report of that, citing multiple passengers' and crewmembers' testimony. Should we have heard early articles saying "a couple of Muslims were thrown off a plane!" and said no more evidence is necessary; it's clearly the work of abusive, racist, anti-Muslim guards? (And no, I don't have reason to think Watts was nearly as alarming as the imams.)