The Great Law
That Which is Above, Remains Above.
That which is Below, Remains Below.

The law is that angels and demons may not take direct action in the world of mortals. But like any law, there are ways around it. Those Above and Those Below can manifest their supernatural essences in fairly subtle ways. Possession is the classic Hollywood approach. But any representation is possible. Celestial emails. Abyssal telemarketers. Shadow puppets counsel heavy metal teens behind the teacher's smoking area. Messages from beyond appear on your TiVo suggestions.

The Devil didn't build a robot but some Korean-American engineer at CalTech did.

"Summoning" is a direct approach that involves a mortal requesting audience. You usually need to know someone to do this and there's no guarantee that your summon will be answered. The common mis-conception is that they're compelled to show up. Not true.

Then why do it? Why make an appearance on the material plane just because some fleshling throws around some dust and candlelight? The same reason evangelists and salesmen will never refuse an invite to someone's home.

They want to sell you something. And they figure that there's a chance you'll buy it.