What zorkian and yvi said, basically. There's no particular reason "why them and not some others" - you just gradually get to make friends. You're not pushing words on anyone by subscribing or posting, and if they really don't like your comments they can always ban you. Also, public entries tend to be public for a reason - if people don't want strangers commenting, they'll turn off comments for people not in their circle, or post access-only.

Most people are uneasy getting too personal with total strangers - that's why public entries are less personal than access-locked ones.