It's quite simple, this very cheap and competitive software is affordable by consumers. And if you run two boards, you need two licenses.
What you could do perhaps is point the 5 domains to one unique single installed instance. So it has the same main url, the same header, users, and content. But the other 4 domains alias to it (location bar rewrites it to that one main one).
And then because of user contributions or ad-income you can invest in additional licenses and grow the network where those alias domains become their own.
Or
Create a business model for your site, where buying 5 licenses up front is an investment you can earn back within reasonable time.
At least since the stable of 1.x there is an option for bulk pricing. And a generous one that is.