Asking Greenpeace to give up their boats would be the equivalent of asking the US to give up our WMD. Oops, I forgot, we don't have any WMD, only the bad guys have WMD. We have defensive countermeasures, but I digress.
The boats are one of their weapons Greenpeace uses in the war on polluters, would you ask the oil companies to give up their tankers? Or their offshore untaxed earnings?
Each side will use the weapons available to them, something about all is fair comes to mind. I'm certain that if the oil companies could devise a product that was non-polluting then the Greenpeace boys would park their boats, and make nice.