Unidan is the exception, not the rule. I think that the number of posts and number of competent admins are so out of balance that they can't catch companies like 'HackPR', or even just regular clever people. Maybe they have a few algorithms to catch the really obvious manipulation, but you can work around IP problems with a VPN, work around 'new, no karma' accounts by reposting from a year or two ago, et cetera et cetera.
I agree unidan is not the rule but there have been many others like him who use sockpuppets that get caught pretty quickly. Happened to a high karma EarthPorn mod who apparently only upvoted himself 4 times. You simply do not hear about them because they are not reddit famous. I used to be very active in reportthepammers so have seen many fairly sophisticated tricksters and spam/upvote rings that got caught. Coordinated new, no karma accounts are easy to spot. It gets much more complicated than that. My main account, which I used to mod a couple of defaults and a bunch of others with and that's about it publicly, was banned after I started privately raising shit about the way they were treating employees. Not even vote manipulation. They didn't shadowban me but changed my password on that and a couple of accounts I was using through different IPs (but were obvious sockpuppets). I have never heard of that happening before or since. I didn't mind though; I was just pushing them to see how far they would go instead of quitting quietly. So it can happen to anyone.