I think the breeders lurk, register, and/or pose as Childfree - or even post as their bitter selves, on the childfree sites for a few reasons. And this is just my opinion.
1. they like to see what irks us to continue to let their kid do it even more to piss off the general populace
2. they are jealous, they love to live vicariously through the veins of the childfree
3. they're just interested, have no opinions, read and go away
4. they're an angry subculture of breederific women who just love to sit around a Starbucks, bitching about the Childfree sites while unattended Shitford spills coffee on somebody's laptop
5. they read our stuff, truly don't understand it because breeding is the only life they've been taught, and pray for us in church
6. they're SAHMs who have nothing better to do all day, they are social retards who can only socialize via the internet, these are the women whose kids are always getting thrown out of playgroups for being brats; these are usually the ones who lash out with angry, accusatory posts about us being selfish and bitter
On the other hand, any CF'er that reads any of the mommy blogs does it out of pure smugness, like watching a trainwreck, you just can't resist. After reading the mommy blogs, we can sit back, thank our lucky stars for our CFness and go about our peaceful lives.
I think there is FAR more lurking from the mommies on CF sites than there are CF'ers reading mommy blogs. Just read any CF site and there is ALWAYS a mommy posting her vitriole. Rarely the other way around. I for one, rarely visit them except if a juicy tidbit is posted here to read and laugh at.