a)the vast majority of tangs WILL need 6ft of horizontal swimming space just based on full adult size alone. also require a large area for grazing. if you plan on keeping most species indefinately sooner or later you will need at least a 6ft tank. Because the smallest standard 6ft tanks are 100g or more it is assumed a 6ft tank will not be a 20g
for a full size hippo, sailfin and any unicorn tang (Naso,Vlamingii ect) a 125g is not big enough and even 6ft is probably too small unless its a 6ft cube.
b) just because it should have it doesn't mean its gonna get it and cant survive without it. there are plenty of tangs in 4ft tanks that will probably live long lives. People make all kinds of rationals why they can ingore expert oppinion but it just comes down to "I want one so I am going to do it regardless".
c) based on bioload a tang can be maintained in a 30g with proper filtration,maintanence and feeding ( I know I did it for a yr before transfering my hippo). Bioload is not the issue. they may be a high load by reef tank standards but by carnivore standards of comparible size fish they arent that taxing. If you can keep a dwarf lion, snowflake eel and toby puffer together in a 75g you can surely keep a tang in one based on bioload.