Eating the sand isn't really the issue... it's an abrasive material, can get caught up in the cloaca, under the scales, etc. If your snake soaks in the water bowl than slides onto dry sand it'll be covered in it. I dunno, it's one of those much debated topics but it's generally deemed not ideal to use sand as a substrate for captive snakes.
Here's a little discussion on the topic.
Weaners at least.