Why couldn't those peeps just say that both evolution and intelligent design are possibilities, or better yet, teach both of them to us and let US decide? 
Because evolution is science and intelligent design - creationism - is a religious belief (whose posing as science consists of trying to - often wrongly - poke holes within evolutionary theory without specifying any detailed alternative theory). As kara pointed out, a scientific theory has to be testable and evidenced. It also has to be open to disproof. And formed from a hypothesis supported by evidence.
ID meets none of these conditions; partially because it's being pushed by a composite group of fundamentalist Christian groups (because even the Catholic church, for example, accepts evolution as a valid theory - their theological beef would be with abiogenesis) with different beliefs of bible literalism, such that a literal definition would be causing arguements between the flat earthists, young earthists, directed evolution, etc groups. Also it's in their interests never to define ID in scientific terms as it could be disproven; antithesis to what is essentially an attempt to crowbar Christian theology into the classroom.
I remember the last, long thread on this. Someone - I believe it was Stealth, not sure (apols if I'm mistaken) - made several arguements based on quotes from famous scientists. One of these was completely made up, the other was true but...er... made up via cutting out large chunks from a paragraph to give the opposite meaning. And then refusing to acknowledge the many transitional fossils that are known, like (for example) the equine ancestor fossils from (IIRC) South America. and soforth.
That's the level of 'science' we're talking here. Well, really I mean level of ignorance; it's born of intentional misunderstanding, ignoring or dismissing factual evidence. That's fine - for faith. Not for anything even
masquerading as serious science.
Evolution can be taught in biology, creationism in R.E. I'd say that's fair and represents the merits of both as a theory of complex life. This verdict is a victory for common sense and rationality.