There may be many things about Oscar Brand to be ambivalent about, but claiming copyright on a collection of traditional material is not one of them.

Consider the way composers' royalties are paid on both books and recordings. If the collector and/or arranger claims a copyright on a slightly altered version of material in the public domain, the publisher pays a royalty to the owner of that copyright. On the other hand, if the material is identified as "traditional," there is no royalty paid, and the publisher makes more money. The original creators of the traditional material are long gone, and the publisher is not likely to reduce the price at which he sells the book or record, just because his costs are a bit lower. So a collector and arranger has two choices: claim copyright and receive a bit more money, or don't claim copyright and let the publisher keep that extra money. Seems to me that claiming the copyright and collecting the money would be entirely legitimate.

Of course, in a perfect world, there would be some kind of fund that collected royalties on traditional music and used the money to support folklorists and other collectors, or maybe to provide support for traditional musicians in need. But that's not the way the system works.