Nazi Looted Art, Fernand Leger and the Minneapolis Institute of Arts

Last week the Minneapolis Star Tribune reported that the Minneapolis Institute of Arts had restituted Fernand Leger's 1911 painting "Smoke Over Rooftops" to the heirs of Alfonse Kann. The article is here. According to the article, it took the heirs ten years of researching and working with the MIA to achieve restitution.
Why such a delay? Many archives are not accessible, much research remains to be done. And there is very little research into a central figure in Nazi art looting: Curt Valentin. Valentin was mentioned in passing in Lynn Nicholas's excellent and groundbreaking work The Rape of Europa (now a film available on DVD).
Curt Valentin left Berlin in 1937 to establish an art gallery in New York City. The gallery was named after Karl Buchholz, one of the four art dealers appointed by Hitler to liquidate art considered "degenerate" by the Nazi regime. I have included a jpeg image above of a letter dated November 14, 1936 from the Reich Chamber of Fine Arts to Curt Valentin stating the following:
Re: Your letter of 22 September 1936
The President of the Reich Chamber of Fine Arts instructed me to tell you that it would be of no objection to him if you make use of your connections with the German art circle and thereby establish supplementary export opportunities, if [this is done] outside Germany. Once you are in a foreign country, you are free to purchase works by German artists in Germany and make use of them in America. ###
The letter bears a stamp with an eagle clutching a wreathed Swastika and is found in the Jane Wade papers, Archives of American Art, microfilm reel #2322, frame 929.
The collections of the Philadelphia Museum of Art (see provenance of El Lissitzky's Proun 2 here, the Museum of Modern Art (a search of the website finds over 40 results for Valentin including a Beckmann here) and many others are full of works purchased from Curt Valentin. When art dealer Otto Kallir came to the United States from Vienna, Austria in 1939, he went to Curt Valentin's gallery on 57th Street in New York City to pick up artworks safely transported out of Nazi Austria. Boston's Museum of Fine Arts has a Beckmann portrait of Valentin here.
Curt Valentin was also the art dealer of choice for Alfred Barr, the founder of the Museum of Modern Art. As Alice Goldfarb Marquis, author of Alfred H. Barr Jr.: Missionary for the Modern, wrote to the New York Times here,
On June 30, 1939, the Fischer Gallery in Lucerne auctioned 150 items. Many dealers boycotted the sale of these stolen works. One of the bidders was Curt Valentin, a German refugee dealer, who owned the Buchholz Gallery in New York. He was there at the behest of Alfred Barr, director of the Museum of Modern Art in New York, who provided money donated to the museum.
Mr. Valentin bought five works: Andre Derain's "Valley of the Lot at Vers," stolen from the Cologne Museum; E. L. Kirchner's "Street Scene" and Wilhelm Lehmbruck's "Kneeling Woman," both taken from the Berlin National Gallery; Paul Klee's "Around the Fish," pilfered from the Dresden Gallery, and Henri Matisse's "Blue Window," seized from the Essen Museum.
So why don't all of the other museums return the stolen artworks that they purchased through Curt Valentin? Why don't the other museums frankly acknowledge Curt Valentin's status as a Nazi agent from 1936 through the end of the war?

On Saturday, September 16, 1944, the Federal Register published an Executive Order dated May 29, 1944 describing a seizure of enemy property by the Alien Property Custodian (a U.S. official empowered by the Trading With the Enemy Act). The seizure was of Karl Buchholz's property, destined for Curt Valentin's gallery on 32 East 57th Street. The list (it looks like over 200 artworks) includes the following artist: Ernst Barlach, Max Beckmann, Marc Chagall, Edgar Degas, Otto Dix, Werner Gilles, Erich Heckel, Carol Hofer, Heinrich Campandonc, Gerhard Marcks, Otto Mueller, E.V. Nay, Emil Nolde, Gaston Louis Roux, K. Schmidt-Rottluff, Renee Sindonis, Alex Jawlensky, Oskar Kokoschka, George Kolbe, Kaethe Kollwitz, Alfred Kubin, Wilhelm Lembruck, Fernand Leger, and August Macke.

If you wish to have a moment of tranquility in New York City, you can to to the MOMA's beautiful garden. You'll find August Rodin's Monument to Balzac (1898). It was presented in memory of Curt Valentin by his friends.