So did the one they're replying to. They're just replying like-for-like.
It's typical that when someone is arrested for "X action with Y detail" (e.g. buying a knife with intent to kill someone) people who oppose the arrest will only state the X (and for some reason this works). To correct the record when someone says "I don't expect to be arrested for buying a knife", "I do expect to be arrested for planning to a murder" is a correct response.
