Your question pertains to two things:
a) The contract between Illusion and the buyer/owner of the game, and
b) Copyright issues.
a) I don't know the details of the contract / EULA, but it might possibly forbid the user from using the game as an "art platform" to re-sell artwork created by the user himself through the use of the game. If the contract does not forbid it, then of course it is OK as far as the contract goes.
Edit: For the contract to forbid you using Illusion's games as an "art platform", you must really have SIGNED such a contract or at least clearly assented to it.
Obviously, this rarely happens when you buy a game, whether you buy it online and it's delivered electronically or you buy it on a DVD, so it's unlikely there's a contract forbidding you this kind of use.
b) Illusion has the copyright to any figures (characters) which shipped with the game. A buyer using these characters as artwork etc. in another game is a breach of Illusion's copyright and hence "illegal". I do not think, however, that Illusion is making any attempt to prevent this, or to sue such buyers/users for damages.
As for other figures / characters:
If they've been created by the buyer /user him/herself, the copyright is the user's own, and he can freely use them.
Edit: Same as if you create a piece of art using Adobe Photoshop or the like. You are then the owner of this piece of art, and holder of the copyright for it, and
the same goes for a movie you're making using a movie editing program etc. etc.
If they've been created by other users, they own the copyright, and using them is a breach of these user's copyright unless he's gotten permission from them. Again - I don't see such "character creators" actually suing for damages.
Even if a user is using his own created characters, using the backgrounds (rooms, landscapes) etc. created by Illusion is a breach of the company's copyright, unless Illusion has given an express permission.
Edit: So my conclusion is: As long as you do not use material that is copyrighted by Illusion (i.e. characters, backgrounds etc.), you should be free to use it as an "art platform". What Illusion write on their web pages about this question is actually immaterial, since you're only bound by a) any contract you may have made with them and b) the copyright laws. Illusion cannot single-handedly dictate what you may or may not do with their products.
I'm not sure this answers your question, but it should indicate what to check to find out if a given use if a breach of contract or copyright or not. BTW, I have a European Master's Degree in Law, but the copyright laws are quite uniform internationally because they are based on international treaties, while the contracts are just what's written in them. Hence whether you see this from an American, European or Japanese legal perspective shouldn't create a significant difference.