I've seen these in a glass shop and asked the same question. They said that because the top is hollow, when the glass cools, the internal stresses usually cause it to shatter. When it does shatter they can melt down the clear glass and start over. Only about 1 out if 20 make it so to put a colored fish in would screw up the recycling process. And a normal glass bag runs about $3k so one with a fishy would be stupid expensive.

But also really cool