I've been hesitated a while on this... I thought the same way you do...

Practically, we have a SAML logout (for CAS and SAML services) and no logout (for Google and OpenID services).

We could handle different kinds of logout in the ```LogoutManagerImpl``` according to the kind of the service if we really need it.
Conversely, the ```logOutOfService``` method is not sufficient to handle properly the front SLO...

The logic of the logout is more complex than just calling a method which performs an HTTP call : it must be gathered in one place, in the ```LogoutManagerImpl```...