From what I understand this change will retroactively apply to games released in the past as well. I think that's a rather scummy move on Unity's part. "I've altered the deal. Pray I don't alter it further."
And it's not like game devs have been using a free product. They already pay for it through expensive licenses per developer.
If the justification on Unity's part is true, that for each install of a Unity game the runtime environment needs to be downloaded from their servers, then maybe they should look into fixing that rather than nickle and diming their customers for each individual install (customers in this case being the game developers)