ResteasyJackson2Provider not implementing javax.ws.rs.ext.Providers interface

The class "org.jboss.resteasy.plugins.providers.jackson.ResteasyJackson2Provider" does not implement its SPI interface "javax.ws.rs.ext.Providers".
I'm sure there must be a good reason for that but I couldn't find any documentation about it, neither in the official documentation nor in the source code.