It's funny because usually the title gets changed from informative to generic (like product name), but here instead of "Apple - Music" it becomes almost an ad copy.
The thread title normally changes to reflect the title of the article.
In this case, the window title of the page is "Apple - Music", but the page headline, which can reasonably be considered the title of this "article", is "Free, three-month trial of Apple Music now available."
So it seems to me that this change is perfectly in line with HN's normal policies.
