So I some more reading on this and it turns out the whole thing is the Russians fault.
“And yet the name and function of Market Island is intimately interwoven with the Swedish-Finnish border (perhaps because the island isn’t much older than that border: Market probably only emerged from the waves in the 16th century. The 1809 Treaty of Frederikshamn, which confirmed Sweden’s loss of Finland to Russia in the so-called Finnish War, also granted Moscow the strategic Aland archipelago, between Stockholm and the Finnish mainland. By sheer coincidence, as drawn by the treaty’s authors the border, halfway between the coasts of Aland and Sweden, ran straight through Market. Which is when and how it got its name.
In 1885, the Russians erected a lighthouse on the island to aid shipping in the surrounding, treacherously shallow waters. Whether by design or accident, the structure was built on the western, Swedish side of the island. But if the Swedes knew, they kept shtum about it, probably still reeling from what happened the previous time they tangoed with the Russian bear.
The de facto border violation continued after Finland’s independence in 1917, and was only resolved in 1981. The solution was ingenious: rather than transfer sovereignty over the lighthouse to Sweden, or physically move it to the Finnish side, the island’s border itself was modified.
Two conditions needed to be met. The sea border was not to change, so as to safeguard existing fishing rights; and each country should retain a similar-sized share of the island. The result was a complicated land swap, leaving the formerly straight border Z-shaped. The lighthouse is now on the Finnish side, in a new bit of Finland west of the former border. Next to it is a companion chunk of Sweden, east of the original line.
Strangely, no border markers actually mark the border on Market: a series of 10 holes drilled into the rock are all that indicate its twists and turns. That’s because anything less sturdy would soon be eaten up by the frequent storms and inclement winter weather. The weather and sea are so powerful that the island’s very shape is subject to significant change. Every 25 years, a bi-national commission will survey the island and effect any border changes that might prove necessary.
The lighthouse has been automated since 1977, and in its uninhabited state had degraded quite severely, until the Finnish Lighthouse Society invaded the island in 2006. It has since been carrying out restoration work (and guided visits) every summer.”
