[Freak_NL]

What would prevent the bots from using the same system?

The minor cost may be offset by the fact that getting a post through to a site that has almost no spam because of its filter is valuable in itself. If the bot is sophisticated enough that the stuff it posts is hard to distinguish from a human shill or shitposter it may even prefer sites protected with such a system.

Useful for the vintage VIAGRA HERE link dumpers perhaps though, but those can be filtered out with a content filter.