Your problem is that while your button is in the same form as the fields the form is being submited and validated by the button.
There are several options to bypass the validation:
- add immediate="true" to the button:
<a4j:commandButton value="pressme" immediate="true" />
- add ajaxSingle="true":
<a4j:commandButton value="pressme" ajaxSingle="true" />
Personally I prefer the second option due to more adequate and responsive behavior.
(sry, didn't see the post date xD)
a4j:region is another option similar to ajaxSingle, but wraps several components.