I believe I found the answer to this question.
In 1997, MGM filed a lawsuit alleging that Disney failed to construct a MGM-themed attraction within nine years of the date the contract between the companies was signed, which was one of the provisions of the deal. They won, and Disney thus lost the rights to use the MGM name outside of Florida and western Europe. There, Disney had been marketing the proposed Disney-MGM Studios Europe, a park that was ultimately cancelled in favor of the smaller Walt Disney Studios park (in other words, by now they may have lost the rights to use that name there, too). Thus, they are forced to remove referneces to "MGM" from all promotional materials that are sent outside of Florida.