One would understand if US authorities wanted the ships to dock at a specific location so that the freight could be inspected and then transported where it was needed. One would also understand that if those wishes were communicated to the ships and the ships' captains disregarded them that we would impose our wishes on the ships' captains, by force if necessary.
In a nutshell, that's just what happened when Israel's navy commandos intercepted the ships bound for Gaza. Israel ordered the ships to sail for Haifa, where the cargo could be inspected for weapons before legitimate humanitarian supplies could be sent along to Gaza.
The "crews" of these ships - far many more than you'd expect to see on a legitimate cargo ship - set out to provoke Israel into a military response. If they're looking for sympathy, they can find it in the dictionary somewhere between shit and syphilis.
Update: The port Israel ordered the ship to may have been Ashdod, and not Haifa. The first article I'd read said Haifa, but others mention Ashdod, and that's closer to Gaza and seems to be where routine (read, actual) humanitarian aid shipments arrive.