So how does an apology make anything better?
Expressing sympathy (which is generally considered distinct) also has an impact, although that can be less surprising.
Explanations generally consider the preference for a consistent world-view — think “We now all agree it was Brad’s fault, let’s now move on” above scape-goating.
In this case, an apology would mean: users were not wrong to trust a well-known app with their personal information, security professionals who warned the company about the breach were not wrong to follow white-hat protocol; the company should have acted on it. It's not clear in this case that ‘sorry’ is an apology, as much as a show of sympathy. As I wrote in a previous comment, I haven’t seen a single user complain about negative effect (or read much on teenager-oriented platforms) so I doubt the sympathy actually is towards typical users; on the other hand, the head-wrangling on Hackers News, and late apology, seem to point at a real concern among security professionals who see standard lowered. I would assume the largest issue is to have the indifferent callousness of Snapchat management contaminate the tech industry, and its perception by the main public; I would actually relate that to the lack of outrage at Snowden’s revelation, and a couple more recent news on privacy: “Our personal information is violated, stolen? Whatever…” Snapchat would make ‘Silicon Valley’ part of that line. Admitting that this is unacceptable would help regain the general perception that personal information needs to be handled with care, generally is and that large brands can be trusted to protect it (be it from black-hats or unwarranted government officials) — yet again, I'm not sure we can read this in SnapChat’s message.