For those that might stumble over this the answer is because s:button does not do a submit of the form. For that you need to use h:commandButton which, according to page 109 of Dan great book, Seam in Action, is an s:button, except it doesn'
t submit form data. DOH!
I guess this is the problem with coming into Seam without a really strong background in Struts or JSF, a little subtleties tacked onto the end of a sentence like that went right over my head. But now I know. And, if you didn't already know it, now you do too.