I assume you are first connecting to the public network, manually.... Right? Windows 8 doesn't have the magical capability of automatically connecting to any network that exists around you without your knowledge. Once you establish a connection, it's up to you to make sure you don't send important info over insecure networks...
Yes, the Mail app connects to Live Mail via a secure protocol...
I have to assume that if you connect to your favorite Cafe's public network with your iPad and you have the iMail app running that you have the same concerns...
C
