they probably guessed u were an oxbridge candidate, and having looked at your credentials, would have thought there was no point, because they could make more use of the place to someone who might actuallygo there- ie someone who might not be likely to get a higher offer or would accept them as first choice.
a girl i know does 5 alevels, predicted straight A's again, does a LOT of extra-curricular music stuff, got rejected for NatSci at durham, but got a place at camb. we dont know exactly why, but i think its cos they mustve found out that she has the poorest communication skills ive ever seen- she just cant talk to anyone, she doesnt socialise at all- durham isnt that kinda place, whereas cos theres so much work at camb, it wouldnt matter as much. from what uve said it sounds like that doesnt apply to you, so maybe they just got fed up sifting the pile of eligible applicants?
as for the discrimination, i think it is fair, not to rejected from EVERYWHERE else obviously, but theres millions of people who cant get into any uni at all, let alone oxbridge, so they should have that chance earlier on, rather than having to wait till august and go through clearing or whatever