The mentioned delete operations are not working as expected and consistent across the major browsers. This behavior is casing the Track Changes feature to be unavailable and unusable in some cases.
The issue will be fixed in Q2 2014 release.
Update - on Feb 28 2014, Ianko kindly indicated that this issue was being worked on: "This issue is currently being investigated by the developer team. It is planned to be fixed for the upcoming official release of the Telerik UI for ASP.NET AJAX version." This is good news as this is a show-stopper for us and our clients as it is impeding the release of a major new feature in our application.
This is a highly desireable feature for many of our business users that do content editing via our CMS. As it stands, it's effectively unusable ("sorry, you can't use the backspace") and is negatively affecting their perception of the quality of the tool.
We are huge fans and users of the Telerik controls. We are entering a development sprint which is focused on highlighting the strengths of the RadEditor and Track Changes is the centerpiece feature. We do hope and encourage you to make this important fix available soonest.