Actually if the police knock on your door and you open it, and in plain sight there is a bag of pot or some other visible sign of illegal activity, they can walk in THERE AND THEN to search your house...As long as they're standing somewhere they're legally allowed to stand. If they actually come into your front door and look in your house before they see something, that's illegal. If they can see it from the doorstep though...That's perfectly legal.
So far as this subject...I'm personally all for it. There's drug deals all the time on the street I live on...Middle class, nice neighborhood, but one kid lives with his grandparents and is constantly meeting people that drive him around, and drop him off a few minutes later. Never long enough to call the local PD and get a patrol car there in time.
It's not like I can call the cops and say "oh, they ran someone over" or "hey they ran a red light"...They can't just pull them over and take my word for it that they commited a crime that cannot be proven. I can, however, say they just met with a known drug dealer, and drove around for a few minutes. That will allow the police to catch the car as it comes out of the neighborhood, and search the car.