I checked two dictionaries: the Oxford Advanced Leaner's Dictionary (OALD), 7th Edition (2005), and Webster's New World Dictionary (1999).
OALD gives busses as a US alternative spelling, while Webster gives busses as simply an alternative. A similar word is bussing, but it's about the gerund (-ing) form, which has a different word formation and spelling convention. Busing would be formed from buse, which as far as I'm aware, isn't a word. For the plural, there's no reason to double the -s, unlike in the case of the -ing form, from which you can - theoretically - predict which word it comes from.
Okay: so it's clear that it's a US alternative spelling, which may or may not come from a misunderstanding of pluralization rules. We've got a suspicious minimal pair* here: boss and bus that are pronounced almost identically in General American English, with the vowel being a bit longer in boss. Since English doesn't have long consonants in pronunciation (unlike say Hungarian), it's easy to mix up the two, and reach the conclusion that if it's bosses, then the plural form of a similarly pronounced word bus should logically be busses, because the difference is one phoneme. This phonetic similarity is valid for speakers of American English, so I think that's the reason it's a US alternative spelling.
Unfortunately, words that end in a lone 's' are very rare in English. I found some medical and biological terms, some of which are indicated by a 'U' (meaning uncountable) or their plural forms are not specified (meaning they're regular, so no duplication of the 's' letters.)
What's correct and what's incorrect cannot be judged based on logic or a given word's etymology or semantics. As much as to-day was the accepted spelling of today some one hundred years ago, it's not correct today, because language is based on conventions, so if speakers of the given language agree that a certain form is correct, it's gonna be used. If people suddenly start to realize they prefer the until-then alternative spelling of a given word, and start using it in such a way, then it'll become the standard spelling. Some decades later, people will simply not realize that their spelling system now actually prefers the "alternative" or "logically incorrect" spelling. The current poll indicates that buses still maintains its preference over busses, so I don't expect that busses would become the preferred spelling very soon, if at all; not to mention speakers of British and Australian English who pronounce the vowels in boss and bus in a recognizably different manner.
I hope this reply has been informative.
* A minimal pair is a pair of words that differ in one phoneme only.