We read that with raised eyebrows this morning while eating breakfast in the LHR Hilton? Where was that warning sign? On a pat of butter.
Who would be the target audience for such a message? First, it's obviously somebody who reads English. Second, it's clearly for somebody who has concerns either about dairy allergies or about consuming dairy products. Its target audience seems therefore to be literate individuals who are concerned for some reason whether or not food contains milk but are unaware that butter contains milk.
Maybe we were missing something. Aha, this article explains it.