Once again the regs in this case are for all rented accommodation, not just HAP.
A prohibition notice means the property by law can not be rented to anybody. Also a prohibition notice comes with a much higher risk of prosecution by the council.
It's a problem that needs addressing because.....
1) the perception of HAP requiring "impossible" standards is a turn off for landlords.
2) where standards are required - then clarity as to why is needed for landlords. Landlords sometimes feel that the house "been failed for HAP" is better then the one they live in themselves. It's easier to accept a genuine need to upgrade a property where clear reasons are offered.