For one, complying with the law is pretty important. For another, when your livelihood is based on (to a non-trivial extent) your physical characteristics and appearance, having your age revealed, and possibly aging you out of more glamorous roles, is an important protection to ensure.
I have no idea why this was down voted, it seems like it was a calculated decision. Litigation can destroy a company (just ask Gawker). Being responsible, legally with consequences, for monitoring the entirety of your site for age disclosure is expensive. Not only to do it, but the cost of litigation for when you miss one. That's probably a big part of the calculation.
Someone suggested that it's because moderating is expensive, which is true, but IMDB has been moderating their site for a long time. I don't think the cost of standard moderation was a big part in the calculation, simply because they have been doing it successfully for years. I think they are also aware of the value of their message boards simply by tracking page hits.
Having said that, public figures are excluded from privacies generally granted to the public. There are plenty of public records of everyone that discloses their age, even though they are not a public figure. To pass a law requiring IMDB remove age disclosure with consequences is very bad law. Not only that, the people that are the "victims" have the financial ability to litigate. It's even worse if other publications like tabloids are excluded from this requirement.
