There's a crowd who tilt the other way -- if I might possibly have hinted at the idea before you then it's borderline malpractice to not reference me. In many fields it's common to directly reference what appear to be the bigger transitive references then, even if they didn't directly influence this work in particular. I'd personally want to see a twidge more evidence before bringing out the pitchforks.

It's innocent in the sense that it's (usually) not checked out of laziness/complacency as opposed to malicious and intentional citation fraud.

Funny. I read your previous comment (with the ?! ending) as sarcastic. Now I see you were serious. I would be astonished if most authors have actually read even a fraction of the sources they cite.

To be honest, this is probably one of the less unhealthy common behaviours in academia :^)