The only reason they didn't get into it was because it was too far from their core business. But now that they control the hardware, the operating system, and the applications that go on there, advertising to such a captive audience is just too easy to pass up.
Nothing wrong with making a profit of course. But people should stop trying to portray this like iTunes. This is not meant to sell more iPhones. Or even make life easier for the user (that would be no ads). This is meant to make Apple money, plain and simple.
iTunes also exists to make Apple money, even if more indirectly.
I also disagree: removing ads wouldn't make life easier for users; it would make it more expensive as free apps would now cost money.
Apple will probably make money, but I don't know if they'll make lots of money on it. Either way, it's not as if they ever dishonestly claimed to be doing this for altruistic reasons. They aren't Google!