You are exactly right, which is why adults are covered by OPPA (where the C, for Children, is removed): http://en.wikipedia.org/wiki/Online_Privacy_Protection_Act

As in contract and rape law, people below an arbitrary cutoff age are presumed to be unable to give consent (or to even understand what "consent" means). Once beyond that age, the person is considered to be able to give consent, e.g. click "Agree" on a Terms/Conditions form or use a website with a very visible Privacy Policy link.

There is definitely an overlap area between < 13 year olds that are responsible and > 13 year olds that aren't responsible, but how can you tell who is who? Maybe 13 was a line in the sand to make the law practical.

Perhaps we should follow the example set by statutory rape laws: children of age x (where x<13) can only give their personal information to sites run by children whose age is within 24 months of x.

(Yes, I am joking.)

No, no. Stupid and ignorant adults are a wildly profitable resource for corporations, advertisers, and marketers. That's not exploitation; it's capitalism. But the children, they must be protected.