Your body likes having water inside of it, and doesn't like having glass shards inside of it
Actually they are quite safe to eat.
https://www.theguardian.com/notesandqueries/query/0,,-186193...
In small quantities. As somebody already pointed, your body likes to keep its water inside it, it also doesn't like when stuff carries large quantities of unmixed digestive fluids from one part of your body to another.
They are probably quite harmful if large pieces get eaten in non-small quantities. Or powder in large quantities that are not previously mixed with a liquid. But I don't know of anybody that tested that.
You can also ingest up to one cup of gasoline.
You can ingest a litre of boiling oil.
well, i hoped i implied "without major injury". or so i've heard. I don't particularly like the flavor so i have avoided drinking gasoline thus far.
With millions and millions of those things around I’d guess quite a few get ingested accidentally and we don’t hear of them causing problems. It must be pretty safe (well as safe as eating a small amount of sand is).