This doesn’t look like a straight purchase either. They’re selling the development rights and it would still be Egypt jurisdiction.
There was nothing to defend. France didn't control the land. It was inhabited by Native Americans; the "sale" was actually a treaty stipulating that France would not oppose the US attempting to expand westward by seizing that land from Native Americans, and also that France would nominally oppose any other colonial power attempting to seize that land (which never happened).
Me neither, but that's not what happened here. They're "selling" the development of the area, not the land itself. At least as far as I understand the article.
2. Taking Alaska as an example, there was a real risk Russia would be in a war with the UK again. If that happened, Alaska was completely indefensible. SO better get money for it now than lose it later.
Not quite the same thing, but still worth a look.