The specific problem with Maps is that Google _can't_ deep link to other providers except by lat-lng or address, both of which throw away the actual context of the PoI in the common case of searching for PoIs. Well, it could perhaps deep-link to OSM-based maps, since their map data is open... but Google, as a proprietary map maker, probably treats OSM data as radioactive.

They could make available a URI using the geo: scheme, but the hacky ways of including a POI as part of a geo: URI (ie using a query string) feel generally unsatisfactory.