Huh, I dug down and apparently it’s a regional quirk?

Even the Wikipedia article about the game doesn’t mention it in English, but does mention it in Russian.

English:

Before play begins, each player secretly arranges their ships on their primary grid. Each ship occupies a number of consecutive squares on the grid, arranged either horizontally or vertically. The number of squares for each ship is determined by the type of ship. The ships cannot overlap (i.e., only one ship can occupy any given square in the grid) or be placed diagonally.

Russian (translated by yours truly):

The playing field is typically a 10x10 square, on which the fleet is placed. <…> When placed, ships should not touch each other by sides or corners. Some variations may lack this rule.

Also, apparently it is common for the game to have 5-long ships, which don’t appear in the Russian versions.

en.wikipedia.org/wiki/Battleship_(game)

ru.wikipedia.org/wiki/Морской_бой_(игра)