Generally, rich text editors keep the focus in the editable area so the user can create content. I am attaching here a short video of MS Word, for example.
If the user needs to focus the toolbar, they can use F10: https://docs.telerik.com/devtools/aspnet-ajax/controls/editor/accessibility-and-internationalization/keyboard-support/overview.
If you want to see a change in this behavior, I suggest you add your request to our Ideas and Feedback portal (https://feedback.telerik.com/project/108/Feedback/List/Feature%20Request) together with an explanation on how you expect this to operated in detail, and how it should be exposed for configuration. If the concept gets a good traction with our customers, our management will prioritize its implementation.