That’s one of those MySQL “features” where it silently ignores your request to do something because it’s simply not implemented:
From http://dev.mysql.com/doc/refman/5.5/en/create-index.html
“An index_col_name specification can end with ASC or DESC. These keywords are permitted for future extensions for specifying ascending or descending index value storage. Currently, they are parsed but ignored; index values are always stored in ascending order“