There are thousands of clones out there, both for-profit and otherwise, and the vast majority have not been pursued. Trademarks on the shapes of "tetrominoes" are questionable (you can't trademark a purely functional element, though you could potentially trademark a particular stylization of it or use of it in a logo). Copyright on game rules is questionable as well, and whether it'd apply in any particular case would depend on both jurisdiction and any potential fair use claims; there's a long history of cloning games, without using any of the original art or other assets. Pretty much the only bit that's more clearly problematic is the use of "-tris" as a suffix; GNOME's version was renamed from "gnometris" to "quadrapassel" for exactly that reason.

I certainly wouldn't give such aggressive behavior any unwarranted credence by presumptively calling a clone "illegal"; on the contrary, I cheer on creative developments like this.