Why isn't anyone challenging herman cain on his tax ideas?

He advocates a "fair tax". It is not a fair tax like what Ron Paul has talked about where everyone would pay a set amount. He instead sets it at 23% rate and offers probates for low income families. This means that low income families wil pay no taxes, just like in the current system. The problem is that right now the top earners pay something like 39%. His proposal would drop that tax rate down to 23%. That is a massive tax break for the rich, without any plan to make up that lost revenue. I support Ron Paul, but I Am not above listening to new ideas. Cain's plan however seems to just replace the IRS with a system that does the exact same thing but taxes the wealthiest by about 40% less. I am all for lower taxes for everyone, but his plan looks like disaster. Why is no one calling him out for it?