Moriendor has the right of it.
From initial ship date (May 2005?) to November '06, BioWare's hands were tied.
As of now, MS can choose to match an offer to BioWare in order to supercede that offer to BioWare.
English: Until November, we couldn't do anything. Now, we have to run it by Microsoft, and realistically, that meant that they could keep us off the PS2/PS3.
The exclusivity deal is essentially Microsoft paying BioWare a whole bunch of money because they're betting that BioWare can convince people who don't already have an Xbox to get one.
Jade sold fairly well -- not a blockbuster, but also not a failure. Its sales look a lot better when you factor in the idea that Jade came out at a point where a lot of hardcore gamers were no longer buying Xbox games in preparation to get the Xbox 360.
As for the question of whether only putting out a product on one platform is unethical or bad or nasty... my brief take is: No.
If I want to watch Rome, Deadwood, or the Sopranos, I have to pay for HBO. If you wanted to play Jade, you needed to pay for an Xbox. HBO doesn't just get good shows because they like 'em (although if they like their jobs, that's a really big plus). HBO gets good TV to convince you to pay more money for it.
I've been in a few meetings where people discussed the cost of developing a product for multiple platforms. If you're doing 360 and PC, it's ugly. If you're doing PS3 and anything, it's about ten times as ugly, because apparently the Playstation is, uh, more-different from the 360 and the PC than the 360 and the PC are from each other. If you're trying to develop for all three, then Holy God, you'd BETTER have a blockbuster on your hands, because you're sinking a lot of money into that baby.
So as a company, you have to look at the numbers. Microsoft offered "X" for BioWare to keep the game off of anything but the Xbox for awhile. BioWare people probably looked at the cost of making Jade for both Xbox and PC simultaneously, "Y", and how much money they'd get for that PC version, "Z".
Then it's a simple question of whether X > (Z - Y).
(Bear in mind: I'm a writer. That's the dumbed-down version that I understand. I'm sure it's a lot more complex than that.)
Evidently, "X" was more.
On our future products, now that we've merged with Pandemic, we've got more resources to streamline multiplatform development. That means that (Z - Y) is a much larger number than it used to be, so unless someone comes up to us with the mother of all "X"es, we're not likely to be interested.
(Mass Effect being different, of course, since we made that contract back before we were big. I don't know if the plan is for ME2 and ME3 to be multiplatform, or what's involved in the contract there. But I know that BioWare is really tired of being seen as "The guys who don't like the PC anymore.")