Actually, is this page true? If so, and if the contract is still in place, then Disney gets 5% of merchandise sales at Tokyo Disney, presumably for doing nothing but being Disney. If so, then I'm hoping they wouldn't want to hinder those sales...though I can imagine them trying to grab some additional sales for themselves. (Which is just what they did in the US--market to an audience that for the most part wasn't going to be importing Duffy goods from Japan anyway, and try to get them interested.)
Anyway, there are dozens of different (often low-quality) Disney items in our 99-cent stores out here, too. In those cases, I don't think Disney is intentionally trying to undermine the higher-quality Mickey Mouse, Cars, Princess, etc. items available elsewhere (after all, Disney makes money off the higher-end things too!). I guess they must see it as offering items at all price ranges, making sure everyone (regardless of budget) is getting a piece of Disney in their homes, etc.
(In reality, despite any intentions, they probably ARE undermining their own brands...I just assume that wasn't the plan.)