Nielsen is very protective of anything they produce. Whether it's ratings data or market rankings. They have a duty to their stockholders to protect those copyrights. If they let Wikipedia get away with using the market data, they would have a hard time charging someone else for it.
I do kind of like one suggestion made earlier: have the FCC come up with their own markets. Frankly, the folks at Nielsen would then be compelled to use the FCC markets for their own since that's how TV station ownership would be measured. It's also probably why the FCC doesn't do that. Nielsen would complain to members of congress and congress would step in and overrule the FCC.
I do kind of like one suggestion made earlier: have the FCC come up with their own markets. Frankly, the folks at Nielsen would then be compelled to use the FCC markets for their own since that's how TV station ownership would be measured. It's also probably why the FCC doesn't do that. Nielsen would complain to members of congress and congress would step in and overrule the FCC.