As far as I can tell, this appears to be a bug - I do not believe that using the buttons in disabled mode should be possible. I have found a workaround though - you can wire the client-side OnButtonClick event handler and return false if the input is disabled - this will cancel the event and the value will not change.
I do agree with you though, that rad Input docs need some work - for example, there is no information about the OnButtonClick client-side event at all (how many parameters it accepts, the type of parameters, etc)http://www.telerik.com/help/aspnet/input/RadInput~Telerik.WebControls.InputClientEvents~OnButtonClick.html
Here is the code I am using to disable the buttons when the input is disabled as well - hopefully this approach will be applicable in your setup.
function ButtonClickHandler(input, buttonName)
<EnabledStyle HorizontalAlign="Right" />