To reproduce: 
1. Drag and drop RadGanttView on the form
2. Populate with tasks and set the ReadOnly property to true. 
3. Run the application and you will see that you can add links

Workaround: 
Disable adding of links:
void radGanttView2_LinkAdding(object sender, GanttViewLinkAddingEventArgs e)
{
    e.Cancel = true;
}