1. He hasn't filed a lawsuit against his accusers
2. He hasn't specifically denied the charges. Cain has said things like "I haven't harassed anyone".
This is different from:
"I was never in a car with Ms. Bialek"
"I never touched Ms. Bialek".
or even better.
"Ms Bialek is lying."
To the public:
"These are false allegations" is equal to "Ms. Bialek is a liar".
but legally they aren't. Cain is using lawyerly words to obfuscate.
If Cain attacks the accusers character, he would be subject to a libel lawsuit.
BTW. If he does say these and none of the accusers sue, then I would tend to believe Cain unless there was a good reason to wonder why someone would allow another person to blatantly lie about them.