right, but the browser doesn't know that and so it has to treat the operation as if it were not idempotent (i.e. warn on a resubmit)

You second paragraph indicates that you do not like the REST pattern of the web, which is fine, but i hope you can appreciate that some of us would like the web as the web to make it possible to abide by that pattern

the last point is addressed in Alex's proposal to allow buttons to function as stand-alone hypermedia controls