Ah. After googling it, apparently the spelling is 'whiskey' in America and Ireland, and 'whisky' in Canada, Scotland, and Japan. So maybe the Maker's Mark brand is owned by Canadians, Scots, or Japanese? Or maybe they like the cachet of 'whisky' vs 'whiskey'? Or maybe they just don't care how they spell it.
Eta: Apparently Maker's Mark is one of the few American companies to use the Scottish/Canadian/Japanese spelling 'whisky' on its product. I found one web page that mentioned this is done to honor the founder's Scottish heritage; it may refer to Burke, who sold the distillery to Samuels in 1953.