Because it invites people to game the system. There are plenty of free lists and they overlap significantly. I'm pretty sure their intersection is also very similar to the core of the HN list.
You can do A/B tests to figure out what sorts of behaviors ride the line of triggering bans vs what kinds of behaviors result in an outright ban. It lets you optimize for just-trash-enough to not get banned, which is not a desirable end state for users.
