Think about it this way.
X = number of users just posting like normal on other social sites.
Y = number of users making low effort posts to farm rewards.
Z = number of users who need to be downvoted when their low effort posts get big upvotes. Without default cap, Z = X + Y.
If X don't bother to change their defaults or have only modest increases on default (the norm on d.buzz), then Z = Y, and Z < X + Y. That means it takes less effort to police Y, because there is no need to downvote X as well.
Y also lose plausible deniability, they can't claim they're being bullied off the platform for no reason.