In relation to the actual discussion and the points posted by mysterycat (which I agree with) I think membership of UKIP has been taken to suggest that the carers would be likely to hold negative views of the family background of the foster children, and their birth parents. The social workers appear to have raised their concerns with the foster carers, and possibly had them confirmed rather than allayed.
Foster carers aren't just looking after children completely stripped of context and what has happened before - even adoptive parents may not necessarily do that now. This was an emergency placement, and most children who are newly taken into foster care will continue to have some sort of contact with their birth parents and/or perhaps other extended family, if that is considered to be in their interests.
If you couldn't care for your own children for whatever reason (and I really hope this hasn't or doesn't happen to anyone here), I don't think you'd want them to be cared for by people who would teach them to be prejudiced against you or anyone from your, or their own, background/race/country.
Also, foster carers by definition, and especially those taking on emergency placements and newly in care children, need to be able to work with birth families - sometimes facilitating contact will be a major activity for foster carers. It's not really going to be a good start if the foster carers are campaigning against the rights of the birth parents to be in this country, or expressing prejudice against people generally from that background.
As for children from religious families being placed with atheists, I quite agree. In the unlikely event I became a foster carer, I would be worried about giving an appropriate upbringing to children from a strongly Christian or Muslim upbringing - and a lot of devout Christians in my area (probably most) are from an African or Caribbean background - although I do have Irish Catholicism and Anglicanism in my upbringing, and I had adults who played an important part who had been brought up as Coptic Christian and Welsh Chapel too, and my partner is from a strongly Methodist family, I really don't think we'd be appropriate....