I don't see how that is another issue. Either they get rid of your data or they don't. If you've ever worked with data at that scale, you'll know how hard it is to keep any sort of promise to actually purge it from all systems.
When I delete my account, any files (photos) I have uploaded are deleted. I don't care if I tagged someone in another photo which I didn't upload, it's their name there, not mine. If a message I sent is in someone's inbox, I obviously don't have any control (actually it depends, but let's take it this way).
It's like if I deleted my email account, I expect my inbox to be deleted, not an email I sent, which is in someone elses inbox.