As with all of the various "Do Not Track" signals (browsers, phones, etc.), they often end up being another data point to be used for tracking, rather than preventing any tracking.
Something I appreciate about Apple's attempt here is the precise phrasing of "Ask the application not to track". It makes it clear that the onus is on the application to abide by it, and it's a not so passive, passive-aggressive dig at exactly these sorts of practices. Apple probably has no real way to enforce true prevention of tracking, but they can make it very clear which side in this war is the one without scruples.
