Cause PRs aren't presented very upfront to visitors and are far less frequently abused. A lot of the open source world relies purely on goodwill, just like the author says.
Also, blogs are on your own domain name usually, which at least gives the illusion that it's your own website rather than just your little tenancy on a blog platform. Some platforms don't give you a domain technically but a / instead, like FB Pages and Reddit, and they explicitly assign you the responsibility of moderating your own page. If their own moderators have to step in, it often leads to deletion.
Also, blogs are on your own domain name usually, which at least gives the illusion that it's your own website rather than just your little tenancy on a blog platform. Some platforms don't give you a domain technically but a / instead, like FB Pages and Reddit, and they explicitly assign you the responsibility of moderating your own page. If their own moderators have to step in, it often leads to deletion.