Ah, you may be right. Back in the day, knocking on the door, dressed smartly, washed and shaved, and asking politely 'could you take on another man, sir?' normally got a positive answer, but I suppose that was a different era - and, of course, when I was getting my first jobs they weren't the sort that would have websites anyway; my first was as a general-purpose dogsbody with a building crew and my second - and my last civilian job - down a mine! Suppose different trades have different methods of entry.
I presume the 'ask the employer' still holds if he has 'hiring now - enquire within' on his door, however? I've seen a few places around town in the past few days with those signs up, and can't imagine you'd be told to clear off if you responded directly to one.