While it's true that MS Exchange can delete messages that have been sent in haste that you can recall, these recalls can be overridden by the person that the email is being recalled from. I.e. I'm a sender sending to the recipient and I recall the message, if the recipient hasn't already received the message then it's easily recallable, however if it's already in their inbox and they're reading it, and then you recall it, they are promoted to accept the recall. They can also change this in the settings, or it can be changed at the corporate IT security policy level.
It's important to note however that most email users are individuals such as you, me, Joe Bloggs, and every other user on the Internet has a personal address and those are usually hosted with one of the following: their own ISP, AOL, MSN Hotmail, Gmail, Yahoo mail, etc ad nauseam. Most users also use the email client that their computer came with, unless it's a web based interface such as Gmail. There are no such recall features built into these services.
So based on looking at what the average Internet user has, and how they use email, it's safe to say that "People can't delete a message they send from a recipient's inbox" is a true statement. As is "... the way every message service ever invented works" is also true. Exchange added onto the normal way, as did Lotus Notes, and the other few, that decided to build this feature in.