Of course this is designed solely to increase Portal 2 sales and make Valve more money.
Make Valve more money, yes, but there's more to it than just Portal 2 sales. They make money off all the indie games as well. And sales like this are a good way to get more people using Steam. The point is, it's a multi-pronged advertisment campaign, and while it's a good one, that's all it is.
What is not obvious to me is why the adults on this site (and others) immediately respond in such a negative, childish fashion to a marketing initiative
Well you answered your own question right there. Many gamers are getting tired of getting excessively marketed to. Personally, I think this is a relatively innocuous one, so it doesn't annoy me. But I can see how other people might feel more strongly about it.
Do these same people work themselves up into such a frenzy about adverts at the movie theatre, or every time they drive past billboards?
No one's worked themselves into a frenzy here, so that's a bit of a strawman. Do people complain about excessive advertisement in other mediums? Of course. The complaints about ads being added to the beginning of movies was considerable. Complaints about increases to commercial time in tv shows was rampant. Complaints about the increase in billboards on the side of the road are common.