While advertisers could maybe possibly sum up a silly damages claim against the professors, this isn't really what it's about. The researchers aren't asking to be indemnified against damages claims -- they don't fear the ad networks/advertisers. Their fear is that the government would prosecute them with a crime for their research.

Personally I think it's a little unlikely, but criminal prosecution can all be very capricious and dependent on local and regional priorities. And so much the better that their suit against the US might bring about limits of CFAA via precedent.