Legal aspects of modding.

Since the support team has already started writing to me, claiming that using the map editor for Heroes 5 is illegal.

O_o
What about the "consumer protection law"? If they don't allow using one of the built-in features of the game, they should refund the money and pay a penalty...
There's one fundamental thing to understand when communicating with them. All these rules they come up with are just nonsense. Copyright restricts the sale and distribution of content; everything else is just fantasy. Even if such clauses are included in the license agreement (and I don't believe that the Heroes 5 license agreement contains a clause prohibiting modifications after the release of the next game), it formally has no legal force under the laws of any country.