40k is pretty much a total rip-off of everything. Ever.
Double Eagle is a blatant 40k retelling of the Battle of Britain. Does that make it a bad book? Hell no!
Yet GW love swing the IP Hammer when somebody dares to copy them. I believe that could be called double standards
I second that.
It's just, some ripoff are more obvious than others, and some are more "in tune" with the general mood of a work than some.
Like you said, ripoff aren't that bad. I remember that a quite illustrious writers said that in the wrtiing world, "stealing idea" was too often defined as "research work".
But you could ripoff something bad, and make it good. (And vice versa, alas !)