This is the default behavior, for security reasons, the message does not indicate that the user is not activated yet. You can add a message on the login page the notifies the users that their account needs to be activated first.
the Telerik team
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items