The geo-political ramifications of capital letters

In otherwise well-written fics the random capitalisation of 'Bi' (as in, bisexual) has been driving me crazy. Why do people feel the need to capitalise what is quite clearly not a proper noun? Those same people would baulk at capitalising 'gay' or 'straight'. They would be exceptionally uncomfortable if anyone around them were to capitalise 'homosexual'.

( more ranting, some citations and a sausage. )

Yeah, OK, no actual sausages were included. I just needed to rant a little. I somehow couldn't focus myself on my theory exam and turned in quite possibly the most superficial piece of tripe I have ever written. Woe. Ah, well, another exam on Tuesday. I'm sure that the prospect of the funding networks of Al-Qaeda and genocidal rape will inspire me.