That's the difference between posting something in a space people view as public and a space people view as their own personal area. People have much more of a right to be offended about the later. You're offering your users a space to call their own, anything you put in there that offends any of them is an intrusion and leads the user to realize that the control of that space is entirely up to you and not them. When you've offered that space to them and they find out your offer is not what they thought it is, it is understandable they get annoyed.
The reason it's important to talk about the emotions is that the emotional calculations change when the space or message is a personal one as opposed to a public one. It's the difference between someone coming up to you on the bus and asking you if you've called your father and a poster on the side of the bus asking if you've called your father. When you put your message in someone's personal space, you don't get to just say "eh, it's probably only a small fraction" because you're specifically communicating with specific people.
It is important to understand the emotional components of software.
I don't buy that Gmail is or is supposed to be a personal area. There are prominent ads, and Google frequently sends out messages to everyone's Gmail about new Labs features and such. Sure, your Gmail is more personal than, say, the cnn.com homepage, but it's still pretty clear that mass messages aren't off the table.