Most people aren't really concerned about mad cow disease specifically - it's not contagious by the usual means but must be consumed by the cow directly. The real problem is that it might be transmitted to humans as new variant Cretzfald Jacobs Disease (CJD), which is what caused a huge scare in Britain in the late 90s. Although there is some evidence of this occuring, it's shaky and the consensus is that the risk of catching NvCJD from eating infected cow meat is too low to worry about.
Mad cow, CJD and other diseases of that class are caused by protinaceous infectious particles, or prions. These are proteins which are mis-folded and for some reason cause identical proteins to misfold in the same way. This results in the formation of holes in the infected individuals brain, giving mad cow disease it's official name 'bovine spongiform encephalopathy' (holy cow brain, and not in the religious sense). Sheep suffered from a variant of BSE called scrapie long before we noticed it in cows. People from Papua New Gineau used to suffer from a prion disease called Kuru. When a loved one died, they customarily ate their brains to send them off to the afterlife. Several people would catch kuru and then die themselves. The cycle would continue indefinitely if it weren't for us identifying the cause and putting an end to brain-eating.
It should be stopped, but as 2-0-Sixx says - it all boils down to the money. A 1% increase in profits is worth the risk (why should the big fish settle with $500 million dollars when they can have $505 million?).