Indeed. It seems shady until you realize why sites do this. Probably the only way to truly understand is to run a site with user generated content for a while.
There will be people using your site that just need to go away. They're different from your typical pharmaceutical spammer because they genuinely think they're not doing anything wrong. If you tell them to stop posting affiliate links to timeshares, they'll put themselves in this box of "banned for doing nothing wrong", and simply start a new account posting affiliate links to timeshares in a slightly different way that hopefully will get around your algorithm.
The worst part is that after enough time, and enough tries, you'l have given them (and the pharmaceutical spammers as well) detailed instructions for how to circumvent your spam protection. All every step of this does is add more work to you to run your site for the people who you want to have there.
So the simplest and most effective thing you can do is make those people think they have succeeded. Let them promote their timeshares (and v1agra) to their heart's content. Just don't show anybody but them.
Real people have friends who read their blog and will point this out if it happens by mistake, and you have a "fix" button in your admin console to hit a few times a year.
Trust me. Run your own blog host for a few months and you'll implement this too.