[MattBlissett]

They're under something like a non-commercial license.

Three parts of a map are copyrightable.

The map data is © OpenStreetMap under their open license, but OpenMapTiles claim copyright on the cartography of their tiles — since they aren't an exact copy of OSM, they decide what to filter for each zoom level, what to simplify for performance and so on.

Additionally, if you choose to use it, a map style is copyright: the design of showing motorways in blue at 4px and cycle paths in green, for example.

For the tiles you see on OpenStreetMap.org, all three parts are under open licenses. (And there are decisions at all three levels, for example OSM's database contains the water features you see at OpenSeaMap.org, but doesn't render them.)

https://openmaptiles.com/terms/