Actually, in the past few years the direction on this issue from Watchtower has flip-flopped.
I remember being told from the platform that if we see a "no soliciting" sign we should respect their wishes and not call on that door.
Simple yes? The brother delivered the announcement with complete conviction. That this is what we should do and it's come directly from Bethel.
Then about 6 months later... we're told to ignore those signs because we're not soliciting. We have sayings of eternal life and would we ignore a "do not disturb" sign if their house was on fire? The same brother gave this announcement with complete conviction.
Then and year later... back to the original idea of not calling on doors with signs on.
The result? Depending on who you work with in the ministry you'll see some ignore the sign and some respecting the sign. Those who were not at the meeting on the day of one of the announcements are convinced that the announcement they heard was the correct one.
So it's made no difference. We're back to where we started. The overzealous nuts will call on the door with signs and the ones that dont like the ministry wont.