That's a legitimate concern, but it has a narrower application than you've suggested. If a blog post doesn't have links that point back to the author's "home" website, then it's not an issue either way. Also, either the author or the guest site can set nofollow.
So our hypothetical innocent user has to be sophisticated enough to 1) have a home site, 2) blog on a different site, and 3) include links, (and have some legitimate non-SEO reason to want to put links to his home page in the first place) while at the same time neither he nor his host be sophisticated enough to know about nofollow.
It's certainly possible but I'm skeptical that it is a large catagory. Plus, I doubt that whatever countermeasure google comes up with is going to be a nuke from orbit response for a very occasional suspected spammy guest post. Instead I expect they will be going after the many many instances that look like a concerted effort.
I must admit that I'm not blogging myself at the moment, so I haven't really thought about these things before now.
My assumption is that a non-technical person would go with some key-in-hand automatically configured solution. So to be on the safe side, any ready-to-use blog hoster should, by default, set all links to be nofollow links, right? Because you really _have_ to know about nofollow if you want to use it only some of the time.
Also, a non-technical person, upon being told that they can't link to their own site and probably also to the sites of other authors on the blog, might not be inclined to see this as anything but some arbitrary rule.
All is probably lost, anyway. This just speeds up the exodus of non-technical bloggers into the walled gardens that lure them.
Well if someone's hosting guest posts on their blog for non-SEO purposes it's generally because the think the guest blogger's writing is good and that their regular readers would be interested in seeing it.
From what I've seen, it's more or less the norm that guest posters have a blog elsewhere that's linked to in the post; if they didn't have one the host would have no way to assess the quality of their posts, and reputable blogs don't accept guest posts from people whose blogs they're not comfortable promoting to their readers.