Ultimately, addons and unlicensed Transformers are probably such a small market in comparison that it doesn't make financial sense for Hasbro to shut them down. To put it another way, it's likely that the estimated increase on sales that may occur is probably a lot less then how much it would take to litigate those cases over years. I have no doubt if Hasbro's execs had a magic lamp, they'd wish that stuff away in a heartbeat, but as of now it's probably just impractical to try and do anything about it. (Aside from edge cases like that Unicron toy they were showing of during Hasbro's crowd funding.)
And that's not even considering thst addon kits can potentially increase sales from people who want the figure, but havec some questions about it the set can fix.