And any utilitarian thinker who wanted to do the op would tell you that they weren't going to get caught, so that outcome won't be relevant to their calculus.
Is that good reasoning? No.
Is it what some humans will do? Yes.
Is that good reasoning? No.
Is it what some humans will do? Yes.
Then they aren't being utilitarian because they are not including the risk into their cost calculation.
Or at least not a good one. Definitely not "any"
edit:
Oh I see this now > who wanted to do the op
Yeah any person can make any false argument to justify a conclusion they wanna reach. You don't have to be utilitarian for that. Just unethical.
Sure they are.
They just estimate the risk much lower than you do.
You have no way to know who's right for sure.
Maybe they legitimately have found a really low-risk way to massively increase utils by cheating / mistreating a few people.
I don't know of a way to protect utilitarian systems from such conditions.