Goodness, where do I begin?
1) She worked so hard to get that title? More so than the general, whom she didn't see fit to address by his title? I doubt that.
2) The general was well within military protocol. Section 4.18 of Army Field Manual 7-21.13 states:
A soldier addressing a higher ranking officer uses the word sir or ma’am in the same manner as a polite civilian speaking with a person to whom he wishes to show respect. In the military service, the matter of who says sir or ma’am to whom is clearly defined; in civilian life it is largely a matter of discretion.3) I've seen some writers claim that the "ma'am" was a result of some kind of misogyny on the general's part. The previous point shows that not to be the case, but in case you needed some more evidence, I would point out that Walsh called male senators "sir" on many occasions.
4) Apparently Boxer had no problem with "ma'am" two years ago, as she called Secretary of State Condoleezza Rice "ma'am" three times. (Of course, at the linked article points out, calling Rice "ma'am" was nowhere near the most offensive thing Boxer said to her during the hearing.)
5) I'm not sure about this, but I believe that a U.S. Senator is nowhere in the hierarchical structure of the U.S. Army. The use of "sir," "ma'am," or even a title for someone outside the military is done as a courtesy, a show of respect. It seems to me that Boxer insisting the general call her "senator" is a lot like a McDonald's manager asking a Burger King worker to call him "manager." The general obliged the senator because frankly, he has more class than she does.
Oh yeah, I promised a funny thing that no one else has mentioned. I guess it's not so much funny as...interesting? Ironic? Inane?
6) My gut feeling is that Boxer didn't care for "ma'am" partly because she is ignorant of the military protocol, but mostly because she thinks of it as a misogynistic term for old ladies. I'd just like to point out that the term she insisted on--"senator"--comes from the word senex, meaning...old man. It's the same place we get the words "senior" and "senile." In contrast, "ma'am" is a shortened form of "madam," which comes from the French "ma dame" and Latin "mea domina," meaning "my lady." It's used (of course) as a respectful term for addressing a woman.
Hmm...perhaps Boxer was right to insist that she not be called "ma'am" after all.