There's more to the story, but only they know what it is.
For some reason or another they didn't want you in that office. It could be as simple as someone else they'd been wanting to hire for some time, decided they wanted to work there after all, and so you got bumped. Yes, they can probably get away with doing that, especially when it comes to probationary, new hires.
All that paperwork you filled out in the morning of your first day. Do you recall signing a piece of paper stating they can terminate your employment at any time for any reason and without notice?
Or, maybe they found some incongruities in your resume.
Who knows? Speculating won't do any good.
I'm really sorry for the disappointment. That just stinks.