The Bush and Blair alliance? Who's the evil one there?
It's Politics.
And different cultural values, and different expectations, and realism and all manner of things which drives nations to make alliances with other peoples making sometimes surprising bedmates. Who would have thought that the USA and USSR could be allies in WWII but they needed to be to rid the world of the Nazis. Who would have thought that the USA and China would ever be allies but they are now, seemingly to flood the world with merchandise (thus proving that its not just desperate circumstances which make for odd bedfellows).
The Rohirrim of course overcame their own innate prejudice against the Woses and ended up making an alliance with them; I don't doubt that the ordinary people of Rohan may have viewed the woodsmen as though they were 'dirty savages' judging by what's said when they meet up. But desperate times forge alliances, and from those alliances often comes greater understanding - looking on the optimistic, non-cynical side of matters...
Perhaps as the Men of Umbar viewed their side as the 'just' side in the war, they overcame their own prejudices against Orcs in order to make an alliance?
As an aside, I would not let the UK off so lightly for their role in Africa.