Some people are wildly speculating that Google and other engines may create historical databases of SEOs and site relationships to identify spam. I have no doubt that some sites that go way too far stay penalized for a long time, and that some penalties may flag related sites for review, but I think search engines have enough data and most people leave enough footprints that search engines do not have to dig too deep into history to connect the dots. And there is little upside in them connecting the dots.
If they did connect the dots manually that would take a long time to do it broadly, and if they did it automatically they would run into problems with false relationships. Some sites I once owned were sold to people who do not use them to spam. If ownership relationships took sites out by proxy I could just create spam sites using a competitors details in the Whois data , or heavily link to their sites from the spam sites.
Where people run into problems with spamming is scalability. If you scale out owning many similar domains you are probably going to leave some sort of footprint: cross linking, affiliate ID codes, AdSense account numbers, analytics tracking scripts, a weird page code, similar site size, similar inlink or outlink ratios, similar page size, or maybe some other footprint that you forgot to think of.
Many of those things can be spoofed too, (what is to prevent me from using your AdSense ID on spam?), so in many cases there has to be a hybrid of automated filtering and flagging and manual review.
And even if you are pretty good at keeping your sites unique on your end, if you outsource anything they are going to have a limited network size, likely a routine procedure with footprints, and if their prices are low they are probably going to be forced to create many obvious footprints to stay profitable. And if you use reciprocal or triangular links associated with those large distributed link farms that puts you in those communities far more than some potential historical relationship of some sort. By linking to it you confirm the relationship.
Search engines do not want to ban false positive, so many spammy link related penalties just suppress rankings until the signs of spam go away. Remove the outbound reciprocal link page that associates you with a bad community, get a few quality links, and watch the rankings shoot up. The thing is, once a site gets to be fairly aggressively spammy it rarely becomes less spammy. If it was created without passion it likely dies then turns into a PPC domainer page with footprints. Hiding low value pages deep in the index until the problem goes away is a fairly safe idea for search engineers, because after a domain has been burned it rarely shifts toward quality unless someone else buys it.
