It's pretty ridiculous, isn't it?

Especially considering that it's lewd & lascivious display of the genitals (and/or sexual activity of course) that is the benchmark of making an image child porn, not topless, or even fully nude. A lot of these sites won't show a totally non sexual topless pic of a girl, or even the slightest nip slip, but they'll show spread eagle in panties or even a thong, which definitely can be considered child porn under US law as well as the law of other countries.