I've used IPB before, I know how the license thing works like you say. But I don't tend to agree entirely with you, because your being forced to keep renewing your license to use the spam tool feature. I was happy to carry on using the version I was back then, no longer upgrading the forum past it any further. So in a sense I had no need to keep paying IPB following the upgrade path any longer, but they force you to do it if you want to carry on using the spam tool. So basically I would have been paying them to use that alone.
Fair enough if your following the upgrade path all the time, but if your not it's a different story then. And what annoyed me is that they've made sure there's no StopForumSpam free add-on as an alternative to fall back on. A few people kicked a big fuss up over it after MATT or Charles removed StopForumSpam from the add-on section on purpose because they started their paid spam tool service.
Worse thing is you can't run IPB without it either, not unless you want a zillion spam bots registering each day.