The words passover, pasch, easter, all basically refer to the same thing from an historical point of view. If some take the position that a Christian festival is intended I think they bear the burden of proving it from context. Why the translators chose to retain "easter" in this one place is not obvious. I don't think it can be shown they deliberately retained it to justify an ecclesiastical custom. They may have thought that the time referent was the most important part of the text, and so they retained "easter" to enable the reader to understand when this occurred. Whatever their intention, if we know that "easter" is basically the same as "passover," we have gained an insight into the time this happened, and our understanding is the better for it. Just as with the weights and measures, or with some regional areas, a more generic term can help the reader to get his bearings where a technical term might be confusing. Translation is better than transliteration in this respect.