Unless they are specifically using names or designs from what they are ripping off, it is considered legally distinct and is therefore lawsuit-proof. The precedent was established LONG ago, and has held up many times.
Shit, Nintendo ended up having to buy the rights to Super Horn Brothers to suppress it, as it was legally a parody and they weren't able to sue over it.
Yes, that is right, Nintendo, beloved by 5-year olds everywhere and emotionally stunted adults, owns a porno starring convicted rapist Ron Jeremy!