You are right. I am a mod of a fairly big subreddit (over 2 million). Reddit really doesn't care.

If the account is new enough and links are spammed within a short amount of time, they are getting auto banned/flagged.

Accounts that are a bit older can get away with it. Sometimes those posts get removed automatically, but that's pretty unlikely. At our sub, we do flag every post that contains a link automatically. In 80% of the cases, this is spam and we delete it.

We have had spammers that kept coming back with new accounts. We have reported it numerous times to Reddit, but we highly doubt that any action has been taken. Especially accounts that try to promote youtube videos are hard to block.

It's generally up to the mods to take action. I do not believe that the 10% rule is enforced in any way.