Because its not just high proteins that cause bloat, it's where the proteins are derived from. Just because it's high in protien doesn't mean it will cause Bloat. Your examples:
Bloodworms: they have a exoskeleton that is hard to digest/process and could potenitally get stuck in the long intestinal tracks of herbivores irritating the walls allowing flagellated protazoas to take hold causing bloat.
Beefheart: this is a mammal protien which contains a different proteins and fats when compared to their aquatic counterparts which the fish is not designed to process. Their digestive systems are well designed so that if the food is of a protein that can not be digested properly it will simply be expell this excess immediately causing irritation again leading to Bloat.
Proteins are made of amino acids which contain nitrogen, carbon, hydrogen, and oxygen, so as the fish is breaking these non-native foods down they in turn make a lot more nitrogenous waste which as everyone knows will effect water quality...another cause of Bloat
Spirulina: now by your definition is to high of a protien(contains 65-75% depending on source) would cause Bloat but does not, again where the proteins are derived from.
The idea of excess protein causing bloat, or any type of gastrointestinal issues in herbivores, is completely inaccurate. Poor quality protein, as well as other hard to digest ingredients, can cause bloat, etc., but easily digestible protein will never cause any dietary issues. I could go on and on but I hope you see the point, percentage means nothing and quality means everything.