This gets weirder. I can't actually access that link because I'm a subscriber to ancestry.co.uk, not ancestry.com, but I've now checked some US census records via ancestry.co.uk and they do display the source citations. But I've re-checked the UK census records and the English parish records - and still no source citations. What I find really strange is that no one else is reporting this problem - it can't just be me, surely!
I've now reported it to Customer Support.