You can set a bit such that the phone will only download new purchases over wifi.
And how many users do you think knew that they needed to do this prior to the album being released?
The autodownload feature is only useful if you're completely within the Apple ecosystem and regularly buy songs from your iPad and want them to show up on your iPhone (or vice versa). If you either never download tracks off the music store or only download them on a single device (such as, say, someone who only owns the one iPhone), you'd never know that there even was an automatic download feature. (And if you're downloading them to your PC, you just sync to it and don't worry about downloading it a second time.)
If you aren't aware there is an automatic download feature, how are you supposed to know to either turn it off or set it to wifi only?
U2 showing up on the phone is likely the first time the majority of users even were made aware that there was an automatic download feature. Based on the articles and comments I've read, I know that there are people who only discovered this feature existed when U2 suddenly showed up on their iPhone. It may turn out to be a vocal minority, but this definitely was not something all iPhone users expected to happen.