"To deem" is an action of consciously deciding. It is an action taken by a sentient being. So to say that some factual evidence "deems" something is nonsensical, as facts are merely ideas, not people or beings with the capacity to make decisions.
It would perhaps be more appropriate to rephrase:
'A Japanese' implies the Japanese person is a thing, and not a person. This is what caused certain people to deem it offensive.
(Note that I also changed infers to *implies. This is the subject of another question, but please research it if you don't know why I did that.)
EDIT: Ah, I just saw the reference to the other question, re: infer. And then I answered the other question. So... go check that out.