Sure, but this is the same as having a locked door with a window next to it that anybody who knows how can reach through and unlock the door. You can say that

Sure, but this is the same as having a locked door with a window next to it that anybody who knows how can reach through and unlock the door. You can say that having the locked door is better - it will stop anybody who does not think to unlock the door through the window. It also stops any robots that come past who are programmed only to try the door-handle and move on if it fails. Problems start to come in when you put this security on a warehouse and customers leave their secure items inside having been assured that the door is locked. Clearly, anybody can just reach through the window, unlock the door, and take the secure items and the customer is left wondering why the advertised locked door didn't help them. Setting up a MITM mail server for an ISP that will happily proxy between the real server and the requester is as trivially easy as reaching through an open window. When asked, they could just say that it is for 'security purposes' and it helps with 'caching'. If your email provider is willing to snoop on all your email, they're probably willing to tell a few lies also.