Can't surely be purchasing just the source considering it's...you know, free?
I on the other hand see why somebody might want to purchase. I know that talk about easy conscience won't carry any weight around here, so how about this: suppose you have intention to build a server on a commercial, that is professional basis (in so far as you don't take into account that Zircon files, even if coded from scratch, are still subject to breach of copyright lawsuits).
By paying something for files, you forestall all the bad blood in mir community if you then serve the files on subscription basis or even with just game shop from which you might have enviable income, especially if you attract mir players with professionally run and maintained server and keep it online for longer time than is usual with most servers.
It is one thing to use the free files to make relatively few bobs off them and quite another to make some ZirconChronicles long running server, because in that case you would be painted as devil incarnated by many in this community, and I guess, deservedly. So if you are financially comfortable, I can see why you'd prefer to 'buy' the right to use the files if only to forestall the above attacks. And I also think, there are still some folks in this universe who might pay for 'ease of conscience' (again if they can afford it without them or their family being put on starvation rations).
All this is not to say that the first thing I also was thinking, why buy something that is free for all...
Also not sure about that 'asian devs' thing, because wouldn't those usually just take, I don't see them offering to purchase in wildest dreams.