That makes me extremely upset to hear. They should see what they're doing is wrong and is in fact just hurting the fishes they're trying to sell.It's caused by poor genetics and inbreeding. The people that produce these fish in Asia are screwing the fish up.
They find a nice fish and breed it with its relatives to make a new strain of whatever fish, in this case it's angelfish. The inbreeding creates the new strain of fish but also encourages bad genetic defects like the intestine popping out.
It's a lottery when you buy fish these days. The more inbred, the more chance of weird stuff happening with them.
Feeding fish a varied diet can help reduce this from occurring but it's no guarantee.
Thank you for spreading awareness, I never knew about this.