Yeah, it's fine for them to take inspiration from your design and use it, after all it's not patented or anything, but it's bad karma not to acknowledge and give you a shout out, frwinks. If I were in their position, I'd be more than happy to give credit, in fact I'd feel downright dirty/lowdown/scummy not to. Why would you not unless you had a big ego to feed or wanted people to think you were something you weren't? But I don't have access to their mind so I can't say for sure...