Well, Hully, I suppose it's because there's a slide, a conflation, between holding a view: holding a view and expressing that view: and holding the view and expressing that view to the child.
So a humanist might be capable of fostering a Christian child, but having expressed his humanist views to the social worker, he or she might consider "insincere" his commitment to not influence the child, and might strike him out. There would be a good foster parent lost.
Take civil partnerships and religious marriage for homosexuals. This is a very fluid situation at the moment, changing fast; some people think changing ahead of social morees.
It's a minefield of opinion. I think it's very possible (though perhaps you don't) that someone who believes civil partnerships should not have a religious element could make a very good godparent. The "law" on civil partnerships at the moment agrees with them. A social worker might not. If a potential foster parent expresses a view on this issue, it could be right or wrong, and different again the day after tomorrow, it could entirely depend on the views of the social worker conducting the interview, it's so open to interpretation. That would be a good foster parent lost.
Someone might let slip they read the Daily Mail, and be questioned about views they hold accordingly, and be rejected by social workers who will not find it difficult to find a discrimination slot.
And the more and more cases there are like this, the more people will be put off. Not just the "rampant homophobes" 
but good people who know they can offer a peaceful respite and don't want to find themselves in a maelstrom of accusation and recrimination.
Sorry this took rather a long time to type.