It seems that the original owners are just not as aggressive as some IPs are. I have read in the past of many games that got threatened by nintendo for having a game character that remotely resembled Zelda's Link character. They are very very aggressive when it comes to characters that could pass as Link.

Spry Fox, creators of Triple Town, successfully sued 6waves for creating a clone game called 'Yeti Town'. It used entirely new artwork, but crossed the line by using a nearly-identical UI layout, tutorial popup messages, item descriptions, and even microtransaction costs.

Basically, they didn't just copy the concept/idea (which can't be copyrighted), but they also copied enough actual data (numbers, text, layouts, etc...).

It was a little more serious than that 6Waves were actually working as consultants with Spry Fox. They had seen the game before it was released and also signed an NDA. There were lots of other clones of Triple Town and some were even closer to the original than Yeti Town but they weren't sue.

The only thing here that could be contentious is the name.

It's also worth keeping in mind that, in the absence of a trademark, the *titles* of individual creative works are not protected the way that the work itself is, if I remember correctly. There can be multiple works with the exact same name and the creators do not automatically have grounds to sue each other.