Why does it say ALL images need alt tags?

I can understand how alt tags help, but for some images that are used as part of the sites navigation, for example, a small 10x10 square that is used to draw a curve and placed on the corner of your tables for decoration, why does the validator want alt tags on those? I hate that I cant make the page validate as XHTML 1.0 because my navigation images do not have alt tags and I do not want to give them any. :-( Any ideas to work around this?

P.S. Yes I understand the validator cannot tell the difference between regular images and images used as part of the layout.