I've always wondered why people keep adding the s. Is it marketing because Google Map sounds like a spelling mistake (missing 's after Google) and we're noticing the effect of that other brand, or is there something about OpenStreetMap that makes it seem like a plural project? People don't say Linuxes, Signals, or Androids
I think it's most likely that most of the other commonly used alternatives (most significantly Google Maps but also Apple Maps, Bing Maps, Maps.me) use the plural.