As wth any sport, you must use the proper regulation equipment if you want to take part in competitions. Nobody cares if you use illegal clubs when you're learning to golf or an illegal bat when learning to hit or a motor in your bike as you head up Alpe d'Huez on your own, but use them in a competition and there's a problem.
Cyclocross has certain equipment requirements. They are there to promote safety and fairness. More importantly they are part of the challenge of cyclocross and one of the things that make cyclocross cyclocross and not mountain biking or road biking. In order to increase access to the sport we look the other way in the B and C races and allow riders to use a wide variety of equipment. However, if you compete in the A race (either by your own desire or you are moved there for competitive reasons) then you must use the proper equipment. It's a reasonable requirement for taking part.
Bike rules as passed in 2010 can be found here. It lists UCI rules and our rules for this year. You'll note they've been relaxed to allow both cat 3 and 4 to use non-compliant equipment.