Use attached to reproduce:
- Disable and then enable the RadSateTimePicker
- You will notice that even the checkbox is unchecked the control is enabled. 

Workaround:

radDateTimePicker1.Enabled =true;
if (!radDateTimePicker1.DateTimePickerElement.Checked)
{
    radDateTimePicker1.DateTimePickerElement.TextBoxElement.TextBoxItem.HostedControl.Enabled = false;
}