Yeah, Dash... Kiwi is flamable; but it is also waterproof, which is why you really polish the boots, it keeps trhe leather supple, prevents cracking and keeps it waterproof.
And Storm is on the money when he says that the high-polish shines are more a matter of pride than regulations. In fact, for the first 8 weeks of MP training, you are forbiden from spit-shining boots because you are not a soldier yet and you are not allowed to do anything that makes you look any better than the ate-up, rag-bag, [bleep]ed-up as a soup-sandwich civilian in training that you are.
That's just one of the ways soldiers in a garrison enviroment say "I'm a pro." I know when my unit is in garrison for our summer camp (which is rare) everyone going out in patrol cars brings an extra starched pair of BDU's an polished boots with them in the trunk 'for emergencies'. That way, if they get into a scuffle, or even if the humidity just kills the starch job on their uniforms, they can still look as fresh and crisp for a call at the end of thier shift as one at the begining.
Though it is tough on the washing machine... and the iron.