Having modded/administered forums before, there's usually an option (usually admin-level only) that requires mods/admin to approve new users before they can post. This process, depending on the admin tools, can be just as tiresome as banning one user at a time, or it can allow for more sweeping ban measures. Either way, it takes the issue away from the regular users sight and puts it behind the scenes.
Another thing to bear in mind is that while it appears we're being assaulted by a faceless bot-swarm, there is a human element behind it somewhere. Someone is scouting these forums, at least initially, to test the spam boundaries. Forums that allow anyone to join and post without oversight become heavy targets, which would likely explain the recent uptick. The latest bots also sport a new approach compared to the usual types found here, so it's likely that a new scout stumbled onto this and unleashed everything they had at it once they saw the lax security.
The next part is about their purpose: clicks at any cost. As we can see from both the usual and newer bot types, only the scummiest and scammiest of entities employ these bot-factories. The bot programmers are employed to get people to click to the site. That's how they get paid. English is an afterthought, as they usually either don't speak the language at all (and rely on translation tools like Google) or have just enough of a grasp to fool themselves into thinking they can fool native and fluent ESL speakers. Obviously, these bots aren't fooling anyone here, but that's not the point. The point is to just get people to click on the links. As soon as it pings back to the site in the link, the bot-factory gets the credit and the payment. That's why every part of the posts and profiles are littered with links in every possible field. If they can't fool you into clicking, they're hoping you click accidentally, because any click is a good click to these jackasses.
The solution is as I said earlier: admin need to up the security. Once a few waves of the bots are culled, the human element will notice when a particular forum is lending them no hits, and when that happens, they'll either move on to a new site. It's too much effort for them to want to try to work around the security measures, because they either have to do it on a bot-by-bot basis or expend effort writing new and increasingly complex scripts. It's either to just find a new target that their existing script will work against.
