Why not? And how is it even “unfair”? They want to charge for every copy sold, that was made using, and is still using, their software.
People who compare this to Visual Studio vs. the MSVC DLLs, are forgetting all the privative libraries which charge for every copy they get released with.
Unity is pulling one of those; only because they didn’t before, doesn’t make it “unfair”, just a dick move.