Because what if you're not on Facebook?
I'm not, so currently feel kind of excluded from this group.
Like I said, what if you're not on the Internet to your suggestion of the website?
If your desire was that great to be part of the group, it would outweigh your desire to not be part of Facebook.
As Facebook is free to join, you're not excluded from anything. You've excluded yourself. Very different.
You don't need to be on Facebook to join though, only to join the Facebook group. As @White Horse 44 has pointed out, you can just email him to join.