"Offensive" was a reason given for the move. I'm not arguing against you exclusively.
They only mean the same thing in dictionary-land. "Nepali" is almost universally used for the national language. "Nepali Sign Language" therefore suggests a connection to Nepali. "Nepalese" is almost never used for the language. You do, however, speak of Nepalese languages, and this is one of them. For example, Nepalese linguistics (2003, Linguistic Society of Nepal) speaks of "Nepalese Languages" but of "Nepali" as the national language; A Foundation in Nepali Grammar (Manders, 2007) presents "Nepali" in the context of other "Nepalese" languages; Language Contact and Contact Languages (Siemund & Kintana, 2008) discusses the influence "Nepali" has had on other "Nepalese" languages; Language Planning in Nepal, Taiwan, and Sweden (Baldauf, 2000) does the same. There is a strong tendency to use "Nepali" for the national language and "Nepalese" for languages of Nepal in general; NSL is one of those Nepalese languages, because it certainly is not Nepali. —