The thing is, all of these elements where originally intended solely for use in forms (except button? it was made later i think). Thus, older browsers may not support them without the <form> element as a parent. But then, why does this matter so much? Just include <form> and </form> anyways... they don't change anything the functioning of your page!
However, in answer to scc's fears, there is really no need to worry about the use of these elements for future browsers, since nowadays they are used with client-side scripts more often than with traditional form processors (ie. CGI, perl, etc.). All new browsers will recognize this and thus not require them to be part of an official form. "Insofar as the propositions of mathematics refer to reality they are not certain, and insofar as they are certain they do not refer to reality." -- Albert Einstein