They're not there to defend Greenland, they're there to die if the U.S. invades. The U.S. keeps small numbers of soldier on the border with North Korea for the same reason.
In any case, the point stands: I would argue that France is willing to put a dozen soldiers in Greenland because it is pretty sure that they won’t be killed by the US, as opposed to sending even a single soldier to Kiev where there is a reasonable chance that they might get killed and we might have to do more than just send money and guns.
If the U.S. seized Nuuk, and Europe mustered a counter-invasion, it would be comparable.
How is that obvious? Unless you have invested in a post-WW3 world.