The original intention behind the turbo button was to give a way to set the clock speed something closer to a 4.77 MHz Intel 8088 for the benefit of games that relied on CPU cycle timing. Therefore turbo was the default and slow mode the exception.

For some reason this feature persisted in PC compatibles long past having any useful purpose, e.g. toggling a 386 between 33 MHz and 25 MHz. Perhaps manufacturers feared any PC without such a button would be perceived as slower, even though as you say, it's really a slow-down button not a turbo button.