The problem is this: never ever ever show customers what they are going to get, and then not give it to them. This is how you get an F in your marketing class. Whether or not it's alpha, is irrelevant. We are not talking about reason, we are talking about marketing and promotion and advertising; this is an emotional issue, not a rational one. If your goal is to disappoint people, then get their hopes up before dashing them. But disappointing people is not a good way to generate sales in the now or the future. You will, however, create distrust and cynicism.
And yes, some are saying this is not just this game, or just Bioware. That is correct, but also irrelevant. The issue is not "is this common practice" but that we were shown a product that does not exist. Some could argue that we bought a product that does not exist, since some bought a product based on promos that did not at all reflect the reality of this game.
The notion that everyone who sees some alpha footage should somehow magically know that this footage is probably not going to be in the game is absurd. Furthermore, to assume that because it's alpha it will likely not be in the game is equally absurd. Never ever "over-promise and under-deliver."
BUT, they did not promise! It was alpha footage! DUH. And once again, you forget that we are not talking about reason, but emotion. Marketing, promotions, advertising, is an emotional appeal, not a rational one. The excitement elicited by the footage does not care about your reason, nor does the disappointment. And those who were not disappointed, why not? Because they assumed that what the footage demonstrated would not be in the game. Why not? Because they have learned to be cynical and distrustful. Why? Because of a battery of very poor marketing practices by developers and publishers that has taught them to treat devs and publishers like liars.