I'd imagine it is incredibly hard, and that entering with the knowledge the arrangement will only be temporary is unlikely to make handing back a child any easier emotionally, but from the article it seems some foster parents have been using the foster system as a means to circumvent the far more onerous adoption system, rather than merely finding themselves alarmed that a child they took in is being returned somewhere they believe to be unsuitable, and willing to fight such a return.

Have no issue with the latter, even if their beliefs happen to be incorrect (that's for the courts to sort out), but the former is an intentional manipulation of the lives of vulnerable children from the outset - before a placement has been initiated, before attachments have formed within a foster family, before the foster parents know any details of the lives of the biological parents & before any indications could have arisen that it would be in the best interests of a particular child to remain with their foster parents.

The whole area is a maze of incredibly difficult decisions, and I don't think there are obvious solutions, other than to remind judges in the family courts to be particularly cautious about the advice of experts & the behavioural theories they invoke.