In my experience, this watering down of the word "atheism" is fairly recent. Five years ago, I think I had never seen anyone use the word other than to refer to the rejection of deities.
And even if, today, it is correct to label a skeptic as an atheist, it is still fact that many atheists actually do reject God.
Indeed, the strong religious forces during the last two millenniums has made their very best effort to polarize the situation, even when no such exist. Still, it is all about basic Greek prefixes. Atheism relates to theism as asymmetry relates to symmetry or acommutativity relates to commutativity. The same has happened to the concept of agnosticism. Due to the influence of religion it is generally though of as a denial of knowledge, even though it is simple the lack of knowledge about deities.
Also, rejecting something does not mean embracing its antithesis. Rejecting the claims of the existence of unicorns does not make you a unicorn denier. It is one of the false dichotomies that religion has tried to pull since the dawn of the entire concept. If we should be extra precise of the definition, it is technically a rejection of theism (belief in god) not deities.
Also, you might be confusing weak atheism on the one hand with strong atheism / antitheism on the other.
Actually, yes it does. It's neither deductive proof nor scientific evidence, but it is evidence nonetheless.
No, since it is a logical fallacy (argumentum ad populum).
This is not a scientific explanation; it is a hypothesis. It's only a scientific explanation if, y'know, it has scientific proof to back it up.
It actually has scientific evidence, believe it or not.
Faces and patterns are very important to humans and all other animals from an evolutionary perspective. It is the way we recognize our group and what is strange. When it comes to errors in pattern recognizing, we can either detect a pattern that isn't really there, or fail to detect a real pattern. To recognize a false pattern will make you loose some energy (at most) with the chance to discover something you can benefit from, whereas the act of not recognizing a pattern that exists can be potentially fatal. This is why, for instance, you are more likely to mistake a shadow for a burglar, than the other way around. Evolution has selected from "superstition". The same mechanism is responsible for the man in the moon, the faces on mars, the religious iconography (virgin Mary on a cheese sandwich, the nun-bun etc.), wearing a lucky shirt during the playoffs, rituals when playing slot machines and so on.
Controlled experiments have also been done to test this. Two groups of people where allowed to play a computer game (one of which rewarded the player in a pattern, the other at random). Both groups claimed that they where given points by a pattern.
http://www.nickyee.com/daedalus/archives/001554.php?page=1
Skinners pigeons is also a classic example of this:
http://psychclassics.yorku.ca/Skinner/Pigeon/
In the ancient world, there was so much that was unexplained and 'purpose' was usually assigned to nature. King Xerxes I of Peria, for instance, sentenced the water to 300 lashes for destroying his pontoon bridge (the ocean was though of as a malevolent force back then).
The evolution of religion / superstition unites a wide range of observations.