The reason is that many Spanish questions can have the same form as a statement and inversion of subject and verb is quite common in statements. Also subject pronouns are not usually expressed. In speech the difference between questions and statements is apparent from intonation. In writing the upsidedown question mark is to warn you a question is coming. I suppose they decided that if they were going to have upside question marks they may as well have upside down exclamation marks too.
Examples:
Usted es my amable. You are very kind.
Es usted muy amable. You are very kind.
¿Es usted español? Are you Spanish?
¿Usted es español? Are you Spanish?
Eres loco. You are mad.
¿Eres loco? Are you mad?