You have to realize that JD very rarely claims their fakes are tamagotchis. It is the eBay seller who sells them and claims they're "tamagotchis;" They can get away with this, most likely, because the seller can say "oh, i was misinformed" without anything really happening to them if they get called on it. Try examining a JD fake next time you see one. There is no bandai logo and there's nothing making claims of it being a tamagotchi - it's the ebay seller always makes the claim. Granted, they do sometimes have Tamagotchi characters and images on the packaging (and I'm not saying JD isn't a nasty company - it is) but perhaps the images aren't subject to copyright or something.
At any rate, Bandai is most likely aware of the fact its toy is being emulated. Heck, maybe they don't even really care. I doubt that JD is taking the market away from Bandai. Bandai doesn't get any money from tamagotchis sold through ebay. Bandai makes money of tamas being sold through stores. And since JD doesn't sell tams through stores (at least, not that I know of,) they aren't really taking any of the market away.
In the end, it's not something they can afford to get their undies tied in a knot over.
It's practically a compliment - JD emulates the tamagotchi phenomenon because really, the tamagotchi franchise is quite brilliant.