It's not free as in you are not allowed to use OSM tiling servers for commercial purposes. "OpenStreetMap’s own servers are run entirely on donated resources. They have strictly limited capacity. [...] OpenStreetMap data is free for everyone to use. Our tile servers are not." [1]. They've even banned some sites for abusing their servers, e.g., FastPokeMap in 2016 [2]. But there is a very clear way of switching to OSM and serving your own map tiles with any styling you want [3]. I personally think that startups like these should go for these options since their user base is concentrated in one region.

[1] https://operations.osmfoundation.org/policies/tiles/ [2] https://github.com/openstreetmap/chef/pull/78 [3] https://switch2osm.org/serving-tiles/