Um, all CDs are mixed and mastered from higher quality sources/recordings. This point is moot.
Lossy AAC is not better than Lossless.
Mastered for iTunes is Apple's way of saying they're trying to enforce some control, consistency or quality in the mastering process such that the resulting balance sounds better or perhaps even suits their lossy format. It's their way of saying this meets our standard for a "good mix". Many pop records are mixed incredibly poorly, driven largely by the loudness war in recent decades.
As with most things Apple it's largely marketing. There's nothing to say producers won't use that "Mastered for iTunes" mix for other CDs or services. I've not found Mastered for iTunes albums to sound better or different than the official releases elsewhere. But I commend their efforts in pushing labels to produce better sounding mixes.
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The Apple catalogue is definitely bigger but I find a good range of fairly obscure stuff I like on Tidal. It's mainly older albums that seem to be missing for some reason.
Don't agree on the Tidal v Apple apps though. Tidal is loads better than iTunes on iOS. I can't stand the new iTunes app. Ugly, too white, text too large. Can't stand the way it mixes my purchased and downloaded content. Hate it.
Mastered for iTunes is actually a specification one must adhere to in the mastering studio not in the mix process. Besides the 1db of headroom for peak to avoid clipping from transient peaks during encoding, Mastered for iTunes specs specify that the AAC encoded audio encoding process occur from a 24bit mastered source. bit depth > lossless for audio quality (so long as the sample rate is a frequency response above human hearing limits.
While the 1db of headroom will largely go unnoticed, the extra resolution in the bit depth will allow for less aliasing in low frequency low volume passages or content.