I'm not sure what you mean by that? We all know the government lies about many things, however in this case they're not saying for one minute that you cannot put A or B cars back on the road.
http://www.honestjoh.../2010-10/cat-b/
http://en.wikipedia.org/wiki/Car_check
http://www.pistonhea...c.asp?t=1339786
The second link may be from wiki so usually of dubious fact, but that's ripped straight from the DVLA/VOSA website. Honest John agrees, there is no legal reason you cannot put a Cat A or B on the roads. It doesn't matter what category the insurer apply to it, it remains your car and a VIC is enough to get a V5 issued, then just an MOT gets you back on the road. The PH link has knowledgeable folks discussing it there too. If you still need further proof, feel free to contact the DVLA and VOSA. It's illegal for a scrappy to sell you a Cat A or B, but not illegal for you to buy and do as you please.
I used to think the same as you did tbh, but then a mate of mine clued me up as he was looking into buying a Cat B MX5. Just because an insurer marks a car as A or B, doesn't stop the car being yours and if there is no payout, you are free to do what you wish with the car. No different from C or D, it's just that it's very rare to find as insurers only usually classify stuff as A or B if there's practically zero chance of a repair ever being made. Doesn't mean they don't make mistakes sometimes though
Anyway, enough thread derailment, I just thought it might be worth looking out for utter bargain Cat Bs for Tarmac too.