It's hard to say whether Apple could even negotiate a deal to offer song sharing. The media companies are so frightened of extending iTunes near-monopoly to other areas that I tend to assume that they'd rather not cut any deals with Apple than let them become dominant in new services.
You're right. However, it's not fair for you to say that they just chose not to implement it. You don't know that. (unless you have some insider info)
While yeah it's not like they lack the ability to add the feature, it's entirely possible that they have been focusing their attention elsewhere and possibly just haven't gotten around to it.
boasting features like song-sharing that Apple still hasn’t managed to catch up to.
Don't be silly. Apple hasn't "caught up," because it was never behind in some kind of race to add song-sharing. Apple chose not to implement that feature, as it has chosen not to add a great many other features.
If you think it's a benefit, it's appropriate to praise Microsoft for adding it, but you shouldn't imply that Apple is staffed by hapless engineers that are somehow incapable of figuring out how to implement this feature, or that Jobs sends out a weekly email asking when this feature will be ready.