Apple was aware of the privacy challenges of this, and laid out the rules for using IDFA in this WWDC 2014 presentation. https://developer.apple.com/videos/play/wwdc2014/715/
It's a video with no transcript, but if you click the link to the slides, you can see on slide 7 that the lifetime of the Advertising ID is "Reset Advertising ID."
Each time you submit an app binary to Apple for review, you have to click a box that solemnly swears that you're using the IDFA to attribute activity to an advertisement, proving that the advertisement did its job. "I, USER NAME, confirm…"
I don't know whether the solemn vows really do anything; I get the impression that IDFA abuse is detected via privacy researchers making noise in the tech press. But it has been enforced a few times.