I'm not 1 million percent sure, but I figure you could probably get away with fitting a presta valve in a rim with a hole for a schrader valve. I know that if you just did that and nothing else you'd get a flat pretty quickly, but there's probably some simple hack to make it work.

As to why one would do that... Lazyness or necessity are most likely. They got a flat and only had a presta tube available...