Postgrest does not adopt this convention. This decision is not arbitrary, however. Postgrest is conceptually a function that computes a REST API for a PostgreSQL database, and tables in a PostgreSQL can (for better or worse) have compound primary keys, which do not map well to a simple URL path. So Postgrest uses a more general URL syntax. The equivalent of the above is: /some_path?id=eq.123.
Postgrest represents some novel thinking about the use of certain HTTP verbs as well. A lot of this is discussed in the Postrest introduction video here: http://postgrest.com/
Those are common API conventions, but they have nothing to do with REST; REST only requires unique URI's for resources, it has nothing to say about the format of those URI's. The Postgres version does not deviate from REST, it deviates from a popular API pattern, that's all.