It's not just the propaganda value the Japanese don't want to give Greenpeace. If the whaler accepts Greenpeace's offer, Greenpeace can hand them a whopping bill as soon as they reach port for a "high seas rescue". The whaler would have no choice but to pay according to international maritime law.
The East Germans learned this the hard way in the 1970s when the West German "Hanseat" (which had been friendly with them for weeks while fishing near Greenland) offered to tow the East German "Brandenburg" to port after its nets got caught up in the screw. Once in port the friendly West German captain handed them a bill for 100,000 DM (West) for the "high seas rescue". (source: "Fänger & Gefangene", ©1983, 1987 Landolf Scherzer)